508 MONTHLY WEATHER REVIEW. NOVEMBER, 1904
Ahsolute teuipzrature.
white sunlight, which, although admitted in small quantity,
increased the percentage of white enough to reach the limit
for z. With a= 8:P 11 a white rainbow was yet obtained, the
white band, however, had already decreased in width.
C‘aluries (grams uf wntrr hrnted lo) pcr +evoud.
RADIATION IN THE SOLAR SYSTEM.
By Prof. J. H. PVINTING, F. R. S.
[dfteruoou address delivered at the Cambridge nieetiug of the British Associatiou for the Hepriuted from Nature, September 23, 1904, Advanc~rneut of Scieuces, Aiigurt 23, 1904.
TOI. 70, p. 612.1 1
I propose to discuss this afternoon certain effects of the
energy which is continuously pouring out from the sun on all
sides with the speeil of light-the energy which we call sun-
light when we enjoy the brilliance of a cloudless sky, nhich
we call heat when we bask in its warmth., the stream of radia-
tion which supports all life on our globe and is the source of
all our energy.
As we all know, this ceaseless stream of energy is a form of
wave motion. If we pass a bean1 of sunlight, or its equiva-
lent, the beam from an electric arc, through a prism, the clis-
turbance is analyzed into a spectrum of colors, each color of
a different wave length, the length of wave changing as we
go down the spectrum from, say, 1/:30,000 of an inch in the red
to 1/80,000 of an inch in the blue or violet.
But this visible spectrum is merely the part of the stream
of radiation which affects the eye. Beyond the violet are the
still shorter waves, which affect a photographic plate or a
fluorescent screen, and will pass through certaiu substances
opaque to ordinary light. Here, for instance, is a filter, cle-
vised by Professor Wood, which stops visible rays, but allo\vs
the shorter invisible waves to pass ancl excite the fluorescence
of a platinocyanide screen.
Again, beyond the rei1 end are still longer waves, which are
present in very consiclerable amount, and can be renclerecl
evident by their heating effect. We can easily filter out the
visible rays ancl still leave these long waves in the beam by
passing i t through a thin sheet of vulcanite. A piece of phos-
phorus placed a t the focus of these invisible rays is at once
fired, or a thermometer quickly rises in temperature. The
waves which have been observed ancl studied up to the pres-
ent time range over some nine octaves, from the long waves
described to the section yesterday by Professor Rubens, waves
of which there are only 400 in an inch, down to the short
waves found by Schumann in the radiation given OE by hydro-
gen under the influence of the electric discharge, waves of
which there are a quarter of a million in an inch. No cloubt
the range will be extended.
Radiant energy consists of a mixture of any or all of these
wave lengths, but the eye is only sensitiye a t the most to a
little more than one octave in the nine or inore.
This radiation is emitted not only by incandescent bodies
such as the sun, the electric arc, or flames. All bodies are
pouring out radiant energy, however hot or colcl they may be.
In this room me see things by the radiation which they reflect
from the daylight. But besides this borrowed radiation, every
surface in the room is sencling out radiation of its own. Energy
is pouring forth from walls, ceiling, floor, rushing about with
the speed of light, striking against the opposite surfaces, and
being reflected, scattered, and absorbed. And though this
radiation does not affect our eyes, it is of the utmost import-
ance in keeping us warm. Could i t be stoppecl, we should
soon be driven out by the intense cold, or remain to be frozen
to death.
As the temperature of a body is raised, the stream of radia-
tion it pours out increases in quantity. But i t also changes
The foot notes to this article, where not otherwise credited, are adaptetl
from a technical paper by Professor Poynting: ‘‘ Radiation in the solar
system: its effect on temperature and its pressure on small bwlies.”
Philosophical Transactions of the Royal Society, Series A, 1903, vol 2W,
p. 925.-F. 0. S.
-
~~
no ..................... 1000. Air Iioiln.. ......... 3009 Earth’s surface.. .. l,(lOOO. Red heat .......... 3, O11Oo. Arc earhou ........ 6 , 0IU” ...................... 6,250”. ....................
in quality. Probably the surface always sends out waves of
all lengths from the longest to the shortest, but a t first when
it is cold the long waves alone are appreciable. As it gets
hotter, though all the waves become more intense, the shorter
ones increase most in intensity, and ultimately they become so
prominent that they affect our sense of sight, and then we say
that the bocly is red or white hot.
The qiiality of the stream depends on the nature of the sur-
face, some surfaces sending out more than others at the same
temperature. But the stream is the greatest from a surface
which is, when cold, quite black. Its blackness means that it
entirely absorbs whatever radiation falls upon it, and such a
surface, when heated, sends out radiation of every kind, and
for a given teniperature each kind of radiation is present to
the full extent,; that is, no surface sends out more of a given
wave length than a black surface a t a given temperature.
A very simple experiment shows that a black surface is a
better radiator. or pours out more energy when hot, than a
surface which does not absorb fully, but reflects much of the
radiation which falls upon it. I f a platinum foil with some
black iiiarks on i t 1)e heated to redness, the marks, black when
colcl, are much brighter than the surrounding metal when hot;
they are, in fact, pouring out much inore visible radiation
tlian the metal.
It is with these lilack surfaces that I am concerned to-day.
But, inasmuch as i t seems absurcl to call them black when they
are white hot, I prefer to call thein full radiators, since they
radiate =ore fully than any others.
For a long time past experiments have been macle to seek
a law connecting the radiation or energy flow from a black
or fully radiating surface with its temperature. But i t was
only 25 years ago that a lam was suggested by Stefan which
agrees a t all satisfactorily with experiment. This law is that
the stream of energy is proportional to the fourth power of
the temperature, reckoned froin the absolute zero, 273’ below
freezing point on the centigrade scale. This suggestion of
Stefan served as the starting point of new ancl most fertile
researches, 1JOth theoretical and practical, and we are glad to
welcome to this meeting Professors Wen, Lummer, ancl
Rubens, who have all clone most 1)rilliant work on the subject.
Among the researches on radiation recently carried out is
one Ly Kurlbaum, in which he determined the actual amount
of energy issuing from the black or fully radiating surface per
second a t looo C., and, therefore, a t any temperature.’
Here is a table which gives the amount a t various tempera-
tures, as determined by Kurlbaum:
Rate ofpow of etier!ly froin one sqtccire centimeter of f u l l y radiating DT ‘‘ black ”
surface.
n. o 0. ln)o1?7
0.0103
1.27
103
I . 6.50
1, Y30
~
As an illustration of the ‘‘ fourtli power law,” let us see what
value i t will give us for the temperature of the sun, assuming
that he is a full radiator, or that his surface, if cooled down,
woulcl be quite black.
We can measure approximately the stream of energy which
the sun is pouring out by intercepting the beam falling on a
~~
[The constant factor, the product uf which by the Pourtli power of the
absolute temperature gives the amouut of radiant energy per square
centimeter pw second, is called the constant of radiation. According to
Kurlbnum, this conrtant, expressed in units of mechanical energy, is
5.32 x 10-6 ergs. By dividing thib by the mechanical equivalent of heat,
it becomes 1.27 x 10-n calories or thermal units.]
NOVEMBER, 1904.
23 iiiillin~i iuilw. _. .. .. .. .. .. . At Mercury's alistauce . . . . . . . At Yenus's alistiruw . . . . ._. . . A t Earth's alistaucg.. . . . . . . . . At -11nrs's distnuce.. . . .. . . . . . At Nrldunr's distnuvr . . . . . . .
MONTHLY VEATHER REVIEW. 509
surface exposed to full sunlight, measuring the heat given to
that surface per second, and then calculating what fraction
the beam is of the whole stream issuing from the sun.
This was first done by Pouillet, and his method will serve
to illustrate the principal of all other methods.
I n his apparatus the sunlight fell full on a box containing
water, and the rate a t which the water rose in temperature
gave the energy in the stream of solar radiation falling on
the box.
Simple as the experiment appears, the determination is beset
with difficulties, the chief being the estimation of the fraction
of the energy intercepted by the atmosphere, and we are stmill
unable to give a very definite value. Indeed, we can not yet
say whether the outflow of energy is constant or whether it
varies. I n all probability, however, i t does vary, and Professor
Langley, who has devoted years of work to the subject, has re-
cently obtained evidence indicating quite considerable variation.
We may, however, assume that we are not very far from the
true value if we say that the streaiu of radiation from the sun
falling perpendicularly on one square centinieter outside the
earth's atmosphere will heat one gram of water 1/24' C. every
second, or will give 1/24 calory per second.'
Now the area of a sphere round the sun a t the distance of
the earth is 46,000 times the area of the sun's surface. The
. energy from 1 square centimeter of the sun thus passes through
46,000 square centimeters a t the surface of the earth. It is,
therefore, 46,000 x 1/24 calories, or 1920 ~a l ./s e c .~ But from
the table already given, a black surface a t 62.50" absolute, say,
6000' C., gives 1980 calories per second, or the temperature
of the sun's radiat,ing surface is 6000" if he is a full radiator,
and there is good reason to suppose that no great error is
made in taking him to be
Let us now take another illustration of the fourth power
law.
3 [ Angstriim estimated this value as 1/15 calory. Langley assunierl that
the atmosphere transmits about 59 per cent of the energy from a zpnith
sun, and from his measuretuent of the heat reaching the earth's surtace he estimated the value of the constant a t lja0 calory per second. Ro-
setti assumed a transmission of 78 per cent from the zenith sun, but
Wilson and Gray consider that 71 per cent represents Rosetti's numbers
better than 78 per cent. If in Langley's value we replace 59 per cent by
71 per cent, we get 1/24 calory.] ( db1Jot, from observations a t the Ah-
trophysical Observatory of the Bmithsonian Institution, obtains a value
of about 1/27 calory per second.-F. 0. 8.)
4 Or, more strictly, if A is half the area of the sun's surface, exprebsed
in centimeters, then 1/24 A calories per square centimeter is the amount
received at the earth from each square centimeter of the sun's surface,
and ---, or I920 calories, is the total radiation from each stluare
centimeter of the sun.-F. 0. 8.
5 [Accepting Angstrijm's value of the solar constant, we should find
7OOUO absolute as the sun's temperature, while taking Langley's \slue it
would be 65000. Wilson (Proceedings of the Royal Society, vol. 69, 1901-2,
p. 312) made a direct comparison of the radiation from the sun with that
from a full radiator a t known temperature. Assuming that the atmos-
phere transmits 71 per cent of the radiant energy received from the sun
when the sun is in the zenith, he obtained 5773O absolute as the solar
temperature. If we denote the solar constant by S, and put 56000 S = 1.27 x IO-'* x 5773'
we get 8 equal to about 1/33 calory per second. This is no doubt too
low a value. Either, then, Wilson's zenith transmission wasless than 71
per cent, or Kurlbaum's constant is too small.
The low value is probably to be accounted for chiefly hy the first sup-
position. Wilson points out that if x is the true value of the transinis-
sion, his value of the temperature is t o be multiplied by (71/z)'~. If we
take 62000 as the true value of the sun's temperature, then x will be
found from the equation
46000 A
24A
This low value is not necessarily inconsistent with the much higher
value, 71 per cent, used in finding Rosetti's solar constant, for no doubt
the transmission varies widely with time and place, and we have no rea-
son to assume that 1.77 calories per minute, obtained by Langley, woulil
have been received from the zenith a t the time and in the place where
Wilson was making his determination.]
Imagine a little black body which is a good conductor of
heat placed in full sunlight a t the distance of the earth. Let
it be 1 square centimeter in cross section, so that it is receiv-
ing 1/24 calory per second.
It will soon warm up to such a temperature that i t gives out
just as much as it receives, and, since i t is so small, heat will
rapidly flow through it from side to side, so that it will all be
very nearly a t the same temperature. A sphere 1 square centi-
meter in cross section has area 4 square centimeters, so that i t
must be giving out from each square centimeter of its surface
1/96 = 0.0104 calorg each second. From the table above it
will be seen that this corresponds very nearly indeed to a tem-
perature of 300' absolute or 97' C., say 70' F.
It is to lie noted that this only applies to a little round body.
A flat plate facing the sun would be about 60' C. hotter, while
if i t were edgewise to the sun i t might be very much colder.
Let us nuiv see what would be the temperature of the small
black sphere a t other distances from the sun. It is easily seen
that, inasmuch as the heat received, and therefore that given
out, varies inversely as the square of the distance, the tem-
perature. by the fourth power law, will vary inversely as the
square root of the distance.6
Here is a table of temperatures of siuall black spheres due
to solar racliation.
1.5UO". .. .. .cart iron iuelts. '3~0.. . . . .~ratl uenrly melts.
210" . . . . . tiu uearl) iiirlt?.
X5O .. . . .slcatht~l hoils freely.
. . . . . . WBI IU LllUllurr day. 9;"
- 80°. . . . . .arctii. COIOI.
-219O.. . . . .uitrugeu frozeu.
We see from t,his table that the temperature a t the earth's
distance is remarkably near the average teinperature of the
earth's surface, which is usually estimated as about 16' C., or
[I n determining tlie steady temperature of any body as conditioned by
the radiation received from the sun, we hare to consider whether i t is
necessary t,o take into account the rdiatiou from the rest of the sky.
If it recreires 8 from tlie s u n , p from t.he rest o f the sky, and if its own
radiation is R, then, in the st,eacly state, that is, when the radiation that
it gives out is equal to that received froin all sources, R = S + p.
It behaves, therefore, as if i t were receiving S from the sun, but as if
it were placet1 in a fully radiating inclosure of such temperature that
the radiation is p. This teinperature is the '' effective temperature of space. *'
The teniperature may perhaps be more definitely described as that of
a sniall full absorber placed at a rlist.auce froin any planet and screened
from the sun. Various well kuowii attempts have been made to estimate
this temperature, but. the data are very uncertain. The fourth power law, however, shows that it is not very much above the absolute zero, if
we can assiiine that the quality of starlight is not very ditfereut from
that of sunlight.
According to 1'Herruite (L'Ast,ronomie, vol. 5, p. 406) starlight is one-
tenth full nioonlight. Full moonlight is variously estimated in ternis of
full sunlight. Langley (First memoir on the temperature of the surface
of the noon, National Academy of Sciences, vol. 31 takes it as io~ooil.
These two values combined give sunlight as 4 x l W starlight, but since
starlight comes from the whole hemisphere, we must, for .the purpose
of comparing temperatures, consider the illumination that would be re-
ceived if the whole hemisphere were paved with suns. This would be
40,UOO x 4 x le = 1.84 X 1W1 times that from the stellar sky. If we as- sunie that the rat.io of the energy of the risible rays to the total energy
is the same in both cases, then, according to t.lie fourth power law, the
el'fective temperature of space is equal to tlie temperature of the sun rli-
vided by
f - temperature of sun.
655 (1.84 x 1U") -
~
Since the temperature of the sun probably lies between 60000 and 701100
on the absolute scale, this gives the effective temperature of space as about 11J3 above t,he absolute zero.
If, then, a body is raised by the sun to even such R sniall multiple of
llJo as, say, 6Uo, the fourth power law of radiation implies that it is giv-
ing out, and therefore receiving from the sun, more than a thousand
t,imes as much energy as it is receiving froin the sky. The sky radiation
may, therefore, be left out of account when we are dealing with approri-
mate estimates and not with exact results.]
510 MONTHLY WEATHER REVIEW. NOVEMBER, 1904
60” F. This can hardly be regarded as a mere coincidence.
The surface of the earth receives, we know, an amount of lieat
from the inside almost infinitesimal compared with that which
it receives from the sun, and on the sun, therefore, we depend
for our temperature. The earth acquires such a temperature,
in fact, that it radiates out what it receives from the sun.
The earth is far too great for the distribution of heat by con-
duction to play any serious part in equalizing the temperature
of different regions. But the rotation about its axis secures
nearly uniform temperature in a given latitude, and the move-
ments of the atmosphere tend to equalize temperatures in dif-
ferent latitudes. Hence, we should expect the earth to have,
on the average, nearly the temperature of the small black
body at the same distance, slightly less because i t reflects
some of the solar radiation, and we find that i t is, in fact,, some
10” less.’
Professor Wien was the first to point out that the tempera-
ture of the earth has nearly the value which we should expect
from the fourth power law.
Here is a table showing the average temperatures of the
surfaces of the first four planets on the supposition that they
are earth-like in all their conditions.
Table of tenipercrtures of earth-like planets.
0 c.
Mercury ............................................ 106
Venus .............................................. 79
Earth ............... 17
Mars ............................................... -38
He has, we know,
a clay nearly the same in length as ours; his axis is incdiiiecl to
the ecliptic only a little more than ours, and he has some kind
of atmosphere. It is exceedingly diilicult to suppose, then,
that his average temperature can differ much from -3s” C!.
His atmosphere may be less protective, so that his day t,em-
perature may be higher; but then, to compensate, his night
temperature will be lower. Even his highest equatorial tein-
perature can not be much higher than the average. On cer-
tain suppositions I find that i t is still 20” below the freezing
point, and until some new conclitious caii lie pointed out which
enable him to establish far higher temperatures than the earth
would have a t the same distance i t is hard to believe that he
can have polar caps of frozen water melting to liquid in his
summer and filling rivers or canals. Unless he is very different
from the earth, his whole surface is below the freezing point.
The most interesting case is that of Mars.
Let us now turn from these temperature effects of mdi a t’ loll
to another class of effects, those due to pressure.
More than 80 years ago Clerk Maswell showed that on his
electromagnetic theory of light, light and all radiation like
light should press against any surface on which it falls.
There should also be a pressure back against any surface
from which radiation is reflected or from which i t is issuing
as a source, the value in every case being equal to the energy
in a cubic centimeter of tlie stream. The existence of this
1 Mr. C. G . Abbot, of the Astrophysical Observatory, in a paper entitled
‘I Radiation ancl teirestrial temperature,” read before the Washington
Philosophical Society on November 12, 1904, disciibsecl the substantial
equilibriuni of temperature of tlie earth, and consequent equality of
solar radiation absorbed in and about the earth to that emitted fioni and
about the earth to *pace. After GpeaLing o f the great complrsity ut’ the
earth and atmosphere as an alisorlier and railiator, certain iuasimriiu
and niinimiun values of the solar con-tant and of the possible terrestrial
teniperature were obtained 1 i y conhidering the substitution o f a Illack
body or peifect radiator for the earth. I n tlih way i t \\ab sho\i.n that the
solar constant can not exceed 3.88 calories per minute, and may be in-
definitely below this according as the earth reflects less than 44 lier cent
of solar radiation, or radiates to space lehs perfectly than a tilack body.
Takiug 1 .9 calories as the ininiinuiii ~llO\%alJk absuniption of the solar
constant, it wab shown that the mean temperature of the earth would
remain above -33’ C. if the earth were a perfect radiator and the reflec-
tion of solar rays did not exceed 44 per cent. Accorcliugl~ we owe not
exceeding 580 C. rise of temperature to the imperfect radiation uf the
earth. (Science, December 9, 19U4, rol. 90, p. 8U2.)
pressure was fully demonstrated incleyenden tly by Lebedew
and by Nichols and Hull some years ago in brilliant experi-
ments in which they allowed a beam of light to fall on a sua-
pencled disk in a vacuum. The clisk was repelled, and they
measured the repulsion and found it to be about that required
by Maxwell’s theory. Nichols and Hull have since repeated
the experiment with greater exactness, and there is now no
doubt that the pressure exists and that it has Maxwell’s value.
The radiation, then, poured out by the sun is not only a
stream of energy. It is also, as it were, a stream of pressure
pressing out the heavenly bodies on which it falls. Since the
stream thins out as i t diverges, according to the inverse square
of the distance. the pressure on a given surface falls off ac-
cording to the same law. We know the energy in a cubic
centimeter of sunlight a t the distance of the earth, since,
moviug with the yelocity of light,, i t will supply 1/24 calory per
second. I t is easy to calcwlate that i t will press with a force
of Gx lop5 dynes on a square centimeter. an amount so small
that, on the whole earth i t is but 70.000 tons, a mere trifle com-
pared with the three million I)illion tons with which the sun
pulls the earth by his gravitation.
Bot now notice the remarkable effect of size on the relation
1)etween the radiation pressure and the gravitative pull. One
is 011 the surface and proportional to the surface, while the
other penetrates the surface and pulls every grain of matter
throughout the whole volume.
Suppose we could divide the earth up in eight equal globes.
Each woulcl have half the diaiueter of the earth and a quarter
the surface. The eight would expose twice the surface which
the earth exposes, ancl the total radiation pressure would be
iloubled, while the total gravitative pull woulcl be the same as
before. Now clilide up e:tch of the eight into eight more equal
globes. Again the radiation pressure ~~o u l d be doubled, while
gravitation woulcl be the same.
Continue the process, and i t is evident that l y successive
division we should a t last arrive a t globes so sinall and with
total surfaces so great that the pressure of the radiation
balance the pull of gravitation. Mere arithinetic shows that
t,his balance would occur when the earth was cliviclecl up into
little spheres 1 /40,000 cm. in diitmeter.
I n other words, a little speck 1/40,00() cm., say, 1/100,000
of an inch in diaineter. and of density equal to that of the
earth, moulcl be neither attracted nor repelled by the sun.
This balance woulcl hold a t all distances, since both would
vary in the same way with the distance. Our arithmetic comes
to this: that if the earth were spread out in a thin spherical
shell with radius about four tiiues the distance of Neptune,
the repulsion of sunlight falling on i t \vould lmlance the in-
ward pull by the sun, and it woulcl have no tendency to
contract.
With further division repulsion woulcl exceed attraction,
ancl the particles would be driven away. But I must here say
that the Inn, of repulsion does not hold down to such fine
division. The repulsion is somewhat less than we have cal-
culated owing to the diffraction of the light.
Some very suggestive speculations with regard to comets’
tails have arisen from these considerations, a i d to these Pro-
fessor Boys directed the att,ention of Secstinn A last year. We
may imagine that the nucleus of a comet consists of siuall
meteorites. When these come near the sun they are heated
aiitl explosions occur, and fine (lust is produced not previously
present. If the dust is sufiiciently fine, radiation may over-
power gravitation and drive it away from the sun, ancl we may
have a iuanifestation of this expelled dust in the tail of the
comet.
I do not, however, want to dwell on this to-dag, but to look
at the subject in another way.
Let us again introduce our siuall 1Awk sphere, and let us
make it 1 sq. cm. in cross section, 1.13 cm. in diameter, and of
NOVEMBER, 1904. MONTHLY WEATHER REVIEW. 511
the density of the earth. The gravitation pull on it is 42,000
times the radiation pressure.
Let
us halve the diameter of the sun. He would then have one-
eighth the mass and one-quarter the surface. Or, while his
pull was reduced to one-eighth, his radiation push would only
be reduced to one-quarter. The pull would now be only
21,000 times the push. Halve the diameter again, ancl the
pull would be only 10,500 times the push. Reduce the diame-
ter to 1/42,000 of its original value, that is, to about PO miles,
and the pull would equal the push.
In other words, a sun as hot as ours and 20 miles in cliame-
ter would repel bodies less than 1 cni. in clianieter, ancl coulcl
only hold in those which were larger.
But it is, of course, absurd to think of such a small sun as
this having so high a temperatme as GOOO". Let us then re-
duce the temperature to ljPO, sity 300" absolute, or the tem-
perature of the earth. Then the radiation woulil he redncetl
to the fourth power of 1/20, or 1/16(l,(JO(), and the diameter
would have to be reduced to 1/16o,UOO of 20 miles, or about
20 cm., say, S inches, when again radiation would balance
gravitation.
It is not rery difficult to show that if we hac1 two equal
spheres each of the density ancl temperature of the earth they
pressure woulcl balance the graritation pull-when their di-
ameters were about 6.8 cm., when in fact these were about the
size of cricket balls.
It must be remerubered that this is only true for spheres
out in space receiving no apprecialde radiation from the sur-
rounding region.
It would appear that we have arrived a t a result of some
importance in considering the aggregation of small meteorites.
Imagine a thinly scattered stream of small meteorites a t the
distance of the earth from the sun. Then, even if they be as
large as cricket balls, they may hare no teudency to move to-
gether. I f they are smaller they may even tend to move apart
and scatter.
I n conclusion, let me mention one more effect of this radia-
tion pressure. You will remember that racliation presses
back against any surface from which it issues. If, then, a
sphere a t rest in space is radiating equally on all sides i t is
pressed equally on all sides, ani1 the net result is a balance be-
tween the pressures. But suppose that it, is moving. It is fol-
lowing up the energy which i t pours forth in front, crowding
i t into a smaller space than if it were a t rest, making i t more
dense. Hence, the pressure is slightly greater, ancl it can be
shown that it is greater the greater the velocityancl tlie higher
the temperature. On the other hand, i t is drawing away from
the energy which i t paws out behind, thinning i t out, as i t
were, and the pressure a t the back is slightly less than if the
sphere were a t rest.
The net result is a force opposing the motion, a force like
viscous friction, always tending to reduce the speed.
Thus calculation shows that there is a retarding force on
the earth as it moves along its orbit amounting in all to about
20 kgm., say, 50 lbs. Not very serious, for in billions of pears
it will only reduce the velocity by one in a million, and i t will
only have serious effects if the life of the earth is prolonged
a t its present temperature to hunclrecls of billions of years.
Reduce the diameter of
the moving body, and the retarding effect increases in propor-
tion to the reduction. If the earth were reduced to the size
of a marble, the effect monlrl be apprecinble in a hundred
thousand years. If it were reduced to a speck of dust a
thousandth of a centimeter in diameter, tlie effect would be
appreciable in a hundred years.
Imagine a dust particle
shot out from the earth and left behind to circulate on its
Now, let us see the effect of size on the radiating body.
would neither attract nor repel each other-their radi a t' Ion
But here again size is everything.
Note what the eEect would be.
own account round the sun. It mould be heated by the sun
and would be radiating out on all sides. As it journeyed for-
ward there would be a resisting force tending to stop it. But
instead of acting in this way the resistance woiild enable the
sun to pull the particle inward, and the fall inward would
actually increase the velocity. This increase in velocity would
increase the resistance, and a t the same time the approach to
the sun would raise its temperature, increase the radiation,
ancl so increase the resistance still further. The particle
would therefore move in a more and more r a i d spiral orbit,
and ultimately i t would fall into the sun. Small marble-
sized iiieteorites would fall in from the distance of the earth
probably in a few milliou gears. Small particles of dust
wo~ild be swept in in a few thousand years.
Thus, the sun is ever a t work keeping the space round him
free from dust. I f the particles are very minute he drives
them forth into outer space. If they are larger he draws
them in. I t is just possible that we have eviclence of this
h i w i n g in in the zocliacal light, that vast dust-like ring which
stretches from the siin outward far beyond the orbit of the
earth, and is at once the largest ancl the inost mysterious
inember of the solar system.
A SIMPLE, EFFECTIVE, AND INEXPENSIVE LIGHTNING RECORDER.
By HEZRY F. ALI I ~T I I K F . 1 Iherier Iuted Noieiulw 22, 19(14
I n the latter part of August, 1902, the writer, a t his own
espense, constructed and erected in the local oflice of the
United States Weather Bureau in New Orlems, La., a light-
ning recorder which has ~J ~O T ed fairly satisfactory. Our ob-
ject mas to olitain automatic records of the hunclreds of elec-
tric cliscliarges, visible and iiii isilde, that usually precede and
accompany thunderstornis, and to study the same with a view
to increasing the accuracy and v-alne of local weather forecasts.
The action of the instrument is basetl upon the effect that
high-tension electric waves in free air, such as lightning, have
upon metal filings huitably arranged in a glass or other insu-
lating tube between two metal electrodes, oiie of which is con-
nected to a collector above the grountl and the other to the
earth. I n their norinn1 state the filings rest loosely a t the
bottom of tlie tribe between two electrodes about ljlt; inch
apart, and their electrical resistance is coiuparatively high.
Nom, when lightning occurs in the vicinity of the filings some
of the electric naves trareliug through the air pass through
the filings from one electrode to the other; this causes the
filing5 to stick together and their electrical resistance is
greatly reduced, thereby rendering i t possible for the current
from a local battery to operate a relay in circuit with the
filings, which in turn operates a (leiice that beparates the
filings ancl restores them to their original condition. and at
the same time records the passage of the electric waves.
Two years' experience with the lightning recorder described
below has demonstrated that lightning records can be used to
some advaiitage in making local forecasts. If, for instance,
the recorder ticks frecluently on a clear slimmer morning when
there are no visible signs of an impending thuiiderstorni (each
tick represents au electric discharge somewhere near the sta-
tion, may be only a mile clistant and may be 50 miles away)
we conclucle that the condition of the atmosphere is unsta-
ble. and that some time during the day there will be a thun-
der5tor111. On July 3, 190:3, for exnniple, the first signal oc-
curred a t 5:21 a. m., and tlie first audible thuncler a t 12:M 1). m.,
or sereu hours and nineteen minutes later. The last thunder
occurred a t 3:OO p. ni., and the last signal a t 3:53 1). m. About
IS0 signals were recorded by the iahtruineiit before the first
audible thumler. I n its present crude condition our recorder
can not tell u s from what direction the storin is approaching
the station, nor with what speed and intensity, but by im-
proving it such information may some day be obtainecl.