MAY, 1898. MONTHLY WEATHER REVIEW. 205
was 0.8’ above the normal. There WBB a slight deficiency over the pan- handle, west Texas, and the central portion of the coast district, while there was a general excess over the other portions Of the State, being slight over the east and west portions of the coast district and ranging from 1’ to about 3O over t h e other portions, with the greatest excess over the central portion of north Texas. The highest was 107O, at Fort McIntosh on the 19th, and t h e lowest, 32’, at Amarillo on tbe6th. The average precipitation for the State during t h e month, determined by com arison of 40 stations distributed throughout the State, was 0.71 below t i e normal. There was a general excess over the panhandle and the western portions of central and north Texas, with the greatest, 4.23, in the vicinity of Brownwood, while there was a general deficiency elsewhere, ranging from about 1.00 to 4.53, with t h e greatest i n the vicinity of Houston. The greatest monthly amount, 7.59, occurred a t Coleman, while none fell at Fort Clark.-I. M. Cline. Utah.-The mean temperature was 53.2’; t h e highest was 94’, at St. George on the 12th, and the lowest, SOo, a t Loa on t h e 311 and at Soldier Summit on the %th. The average preci itation was 3.02, or
considerably above normal; the greatest month& amount, 7.04, oc- curred at Heber, and t h e least, 0.61, a t Fort Duchesne.-J. H. Smith. Vginics.-Themean tem erature was 65.6’, or slightly ahore normal; the highest was 9So, a t Bal~sville and Doswell on the 40th, and the lowest, 30°, at Dale Enterprise on the 9th. The average precipitation
was 5.35, or 0.91 above normal: the greatest monthly amount, 9.08, oc- curred a t Lynchburg, and the least, 3.53, at Biickinghaxn.-E. A. h .n e . Wa8hinglon.-The mean temperature was 55.2, or nearly normal; the highest was 92’, a t Kennewick on t h e 25th, and at Lind on the 26th, and t h e lowest, 24’, at Centerville on t h e 31st. The average precipi- tation was 1.81, or about 0.50 below normal; the greatest monthly amount, 5.06, occurred a t Clearwater, and t h e least, 0.12, at Ellens-
bu#- eel PiTginia.-The mean temperature was 63.0’. or about 1.5’ above normal; the highest was 94’, at Eastbank on the Zlst, and the lowest, 2So, at Beverly on the 9th. The average precipitation was 4.51, or slightly above normal; th: greatest monthly amount, 6.15, occurred at Beverly, and the least, 2.r6, at Parkersburp.--C. dl. A’trong. lViewtiein.-The niean temperature was 55.7’, or nearly normal; the highest was S9O, a t Chat on the 7th, at Knapp on the 23d, and at Prairie du Chien on the 24th. The average precipitation was 234, or 0.55 below normal; t h e greatest monthly amount, (5.60, occurred at Osceola, and t h e least, 1.10, a t La Crosse.-K M. Wilson. Wyoming.-The mean temperature was 48.0’, or 4.3’ below normal; the highest was S9’, at Fort Laramie on the 31st, and the lowest, 15’,
a t Sherirlan on the 6th. The average precipitation was 3.78, or 1.72 above normal; the greatest monthly amount, &(E, occurred :it Lander, and the least, 1.46, a t Bigpiney.- TI’. 6’. Pamcr.
8. N. ,Salimbury.
- -
SPECIAL CONTRIBUTIONS.
MOISTURE TABLES.
By Prof. C. F. MARVIN.
The quantity of moisture mixed with the air under difler-
ent conditions as to temperature and degree of saturation
often plays an important part in the operation of blast fur-
naces, drying kilns, cotton mills, steel mills, etc. The metal-
lurgist, especially, is awakening to the iniportance of taking
full account of the moisture in the air that incidentally, or
designedly, is often a part of extensive chemical operations
involved in the production of steel and iron.
From time to time letters requesting information on these
questions have been received by the Chief of the Weather
Bureau, and i t has seemed advisable to publish a general an-
swer to such inquiries in the shape of the following notes and
table.
The weight of a unit volume of vapor is given in the
revised editions of meteorological tables only for conditions
of complete saturation, whereas, in ordinary practice we deal
nearly always with cases of partial saturation, and i t is
believed the table below will be useful to many and obviate
the necessity of special computations.
Faulty conceptiom.--A false notion that the air has a cer-
tain capacity for moisture is widely prevalent, and is perpetu-
ated by all such expressions as “The air is partly saturated
with moisture,” “Weight of aqueous vapor in a cubic foot of
saturated air,” etc.
It should always be clearly observed that the presence of
the moisture in any given space is independent of the presence
or absence of air in the same space except that the air retards
the diffusion of the vapor particles. It is more correct to say,
in the above cases, that the space is partly saturated with
moisture, or the moisture is partly saturated or is superheated.
By all means use the phrase “Weight of a cubic foot of sat-
urated aqueous vapor,” not “Weight of aqueous vapor in a
cubic foot of saturated air.”
The amount of saturated aqueous vapor that can exist in
any given space depends entirely upon the temperature. It
appears that the vapor may be supersaturated under certain
peculiar conditions, but this is a special and an unstable state
which need not be considered in the present connection.
When the vapor is saturated, it will exert a certain pressure
which varies with the temperature and which so-called “maxi-
mum pressure ” has been measured with greater or less preci-
sion over a long range of temperature from about 60° below
zero F., to far above the boiling point of water.
Saturated aqueous vapor is but little more than half as
heavy as the same volume of air under like conditions of
temperature and preasure, and, in all ordinary computations
i t is assumed that the exllansiion and contraction of partially
saturated aqueous vapor is i n accordance with the same laws
as apply to air and ordinary gascis, which d u not easily con-
dense to the liquid state.
The adopted densityof snturatc 11 aqueous rapor is not deter-
mined directly from experiment, 1Jut is deduced theoretically
from the observed fact that two volunies of hydrogen and one
of oxygen combine to produce two volunies of water vapor.
The weights of unit volunies of hydrogen, oxygen, and dry
air are accurately k~iown, from which the specific gravity of
aqueous vapor is found to be 0.6321.
The weight of a cubic meter of saturated aqueous vapor is
given by the equation:
in which t is the temperature, centigrade, and F the corre-
sponding pressure, in niillinietera, a t saturation. A is the
weight of a cubic meter of air, under standard conditions
= 1.29178 kilogram, k is the coefficient of expansion of air =
0.003667.
If English units of temperature, pressure, and weight are
used, we find the weight of a cubic foot of saturated aqueous
vapor in grains is:
This formula gives the weights found in the colunin headed
‘‘ 100 ” in the accompanying table. Above 3 1 O the values of
F’ employed were those deducecl from Regnault’s observntions,
by Broch, for the International Bureau of Weights and Meas-
ures. Broch’s reduction is unsatisfactory for temperatures
below 32O, and this portion of the table is based upon satura-
tion pressures experimentally observed by the writer and de-
scribed in Appendix 10, Annual Report of the Chief Signal
Officer, 1891.
When the water vapor present in any given space is not
saturated, this fact is generally expressed by the degree of
humidity assigned to it. For example, we say the relative
humidity, that is the percentage of saturation, is 60. This
means that only 60 per cent of the vapor that might a t the pre-
vailing temperature exist in the space under consideration is
present; hence, 40 per cent more vapor must be added in
206 MONTHLY WEATHER REVIEW. MAY, 1898
__
order that the space may be saturated. We may deduce the
percentages of saturation either as a ratio of the weights, or
aa a ratio of pressures, with identical results, because in all
such computations it is assumed without important errors
that partially saturated vapor expands and compresses strictly proportional to the temperature and pressure. From this i t
follows that the weight of vapor at a given percentage of slit-
uration is found by multiplying the weight corresponding to
saturation by the relative humidity.
Waght of a cubic foot of aqumus aapor, et&-Continued.
Percentage of saturation.
__~ ____________
10 I 20 1 3 0
~
40 1 5 0 1 6 0 1 7 0 1 8 0 1 9 0 )l a ,
~~ ~
._________
Grains.
__
5
$6
E?
+l
~
+w 51
52
53
54
55
56
57
511 59
60 61 62 63 64
&?I
66
67 68 6Y
70 71
73 74
75 76
78 i 9
pa 81
83
a
85
86 87 M
8Y
90 91 92 93
81
M
96
97 9H
gY
IN 101 IW 109
104
105 lot 1K I* lo(
m, 8,.
""
+I11
~
__
1.630 1.889 1.749 1.810 1.874
1.W 2.006
2.076 2,148 2.222
2. L 2
2.376
2.457 2 .M
2.625
9.113 2.m 2.8% 2. w 3 .M
3.143 3.296 3.4M 3.513 3 .6 3
3.743 3.882 9.985
4.111
4.240
4.374
4.510
4. 6M
4.795 4.41"
5.094 5.251 5.410 5.515
5.744
5.916 6.094
6.276
6.462
6.654
d.050
7.257 i. 4tB 7.645
7.906 8.134 8. ,367 8.606
8.8.W
9.100 9.357 9.619 9 .W IO. 163
10.445
!. w
~
2.446
2.5.%3 2.623
2.716
2.811
2. Ikw 3.010 3.115 3.?32
3.333
3.447 3. 565 3.685 3. em 3.w
4.069
4.m 4.345
4.4M
4. m6
4. m 4.944
5.105 5.339 5.440
5.614
5. is3 5. m 6.106
6.361
6.560
6.765
5 976
#. I92 7.414
7.6.12 7. 877 8.116 8. W2 8.615
8.874 9.140 9.413 9.693 9.980
10. 274 10.576 10.8135 1 1 .m 11.w
11.w
12.201
12.550
12.906
13.2i5
13.6.50 14.035 14.4%4 14.W 15.245
15.667
~
3.261 3.378 3.490 3. 621 3.748
3.879 4.013 4.153 4. !El0
4.444
4.596 4.753 4.914 5.079
5.m
5.428
5.607 5. 793
5. w 6.181
6. .W
6.592
6.806 7.0% 7.253
7.485 #.,.A 7. !GO 8. 22-2 8.481
8.747 9.020 9.301 9.590 9. WI
10.189 10.w2
10.621 11.150
11.87
11.832
12. 18i
12.551 12.92 13.30;
13.6911 14.101 14.514
14.997 15.370
15.819 10.32 16.734 17.211
17.700
18.W 18.714
19.2.3 19.776 33.M
20.8W
- -.
0. .um 0. $22 0.437 0.453
0.468
0.485 0. m2 0.519
0.597
0.563
0.574 0.594 0.614 0.635 0. u58
0.701
0 cw 0.748 0. 773
0.799 0. Rw
0.%1 0.878 0.907
0.936 0.988
0.996 I . (W
1 .0
1.0%
1.128
1.163 1.199
1.%30
1.2i4 1.313 1.359
1.394 1.436
1.479 1.523 1.569
1.616
1.663
1.712 1.763
1.814
1.86i 1.921
1.9T 2.w 2.cw2 2.151 2. 212
2. 275 2.339 2.405 2.4c2
2.541
2.611
0. 1378
0.815
0. $44
0. M 0.905 0.%7
0.970 1.033 1.038 1.054 1.111
1.149 1.1%
1.228 1.50
1.313
1.356 1 .m 1.448 1.4% 1.515
1.596 l.w 1 . 7v2
1.7% 1 . HI3
1.871 1.931
1 . s??3 2.055
2. IW
2.187 2.255 2.325
2.997 2.471
2.517
2.625
2.705 2. 787 2,872
2.9,s 3. u7 3.18
3.231 3.33
3.425 3.525 3.628 3.7% 3. W!
3.953 4.067
4. 1133 4.w3 4.425
4.550
4 .m 4.914
5.w2
5 .9 2
4.678
1.223
1.267 1.312 1.358 1.406
1.455 1.505 1.557 1.ti11 1.666
1.724 1.782 1.W 1.905 1.961
2. a35
2.172
2 .2 4
2.318
2.3'94 2.472 2.555
9.6%
2. ?a
2. tu); 2. R96 2. sL(9
3.083 3.180
3. Zen 3.38..
9.4M
5.596
3.7E
3. e21 3.[1113
4. w 4.181 4 .m
4.43; 4.570 4.707 4. B1C
4,'M
5.137 5.2% 5.443 5 .M
5.76.1
5.9x
6.1M
6. %?
6.454 6.W
1 .O l t
, . -14
i.4lf
' . b,.
7.834
M. ia3
$.m
- .,
" .h
2.w 2.111 2.186 2. !2w 2.3Y2
2.424 2. xw 2 5% 2.035
2.778
2.872 2. 970 3 .m 3.174 3.'m
3.391 3.503 3.6w 3. 740 3. %3
3.99.)
4.1%
4.254 4.391
4.599
4.678 'I. 88 4.981 5.1.36 5. w
5.467
5.0.38
5. 1113
5.m
6. 178
6.368 6 .W 6.763
,.160
7.3%
7.617
7. a 4 8 .0 3
8.311
8. 56!2
8.813
q.011
9.336 9. m
I). w3 10.168 10.4% 10.757 lI.o(I2
11.375 11.6W
12.DL-l 12.3lx 12.704
13.0%
8,. 968
2.853
2. Y55
3.060 3.16A 3:m
3. 381 3.51 1 3.li34 3.759 3. w3
4.0e 4.159 4 .m 4.444 4.594
4.747 4. m 5.069 5. 5 .m
5.5%
5 .m 5.!156
ti. 147 6.,346
6.519 6.759
d.191 7.421
7. H5L 7. 892
U. 138 8.391 8.649
9. 189 9.468 9.7% 10.051
10. 351
10.664
10.-
11.308
11.644
11.907 12.m
12.e99
13.uio 13.448
13.836 14 '394
14.W 15.06(1 1 5 .W
15.925
16.374 16.834 17.5m
17. 786
18.58
!. %3
8.915
3.068 3. Hoo
3.935 4.073 4.216
4. w 4.514 4.676 4.833 5.000
5.170 5.347 5.- 5.714
5.907
6.104 6.308 6.517 6 .7 1 6.953
7. Id2 7.418
z.657
1.W
8.159
8.690 8.960 0.249 9.541
9.841 10.118
10.4l?4 10.7% 11. 1'2
11.41 11.814
12. 173 19.549 12.922
13.311 13.711 14.M
14.971
15.412 15.W 16.W 16.804
17.291
17.78s 18.30.
19.361 19.91:
SJ .471 21.05'
91.63: 22. M %'.Mi
23.501
n .4 ~
14.540
IR.W
4.076 4.222 4.35" 4.51 4.686
4.849 5.016
5.191 5. EO 5.555
5.745 5.811 6.145 6.349 6.563
6.7M
7.009
4.480
7. c26
7. w 8. w 8. .Mi
8. 78'2 9.W
9.956 9.855 9.96%
10.277
IO. 601
10.934
11.23 l l .B I
11.w
12.w
12.7%
13.181 13.W
13.937 14.369
14.790 15.%14 15. W 16.155
16.831
17.124 17.026 18.142 18.671 19.219
19.7w
ru. Y.35
W.91T 21.514 22.
22.760 23.3%
24.048
2.4. ml !&. 4im
26.112
2241
Weight of a cubic foot of aqueous vapoi. a t diffe~ent temperiitures andpercent-
ages of saturation.
Percentage of saturation.
-
10 40
~
TO 100
+ Grains.
__
0.10 0.lM 0.110 0.118 0 .1 3
0.131
0.139 0.146 0.154 0.162
0.171 0.180 0.190 0.199 0. ,210
0. 222 O.'B'j
0.247
0. LW
0.274
O:B9 0.3aY 0.317 0.332 0.349
0.366
0.403 0.m
0.443
0.466
0.490 0.514 0.539 0.565
0.592
0.61Y
0.643
0.677 0. $09
0.741 0.776 0.813 0.&1 0.890
0.931 0.9.74 1.018
l.w 1.11%
1.161 1.213 1.268 1.316 1.367
1.4W
1.474
1.w
1.w 1.w
1.709 1.773 1 .8 s 1.m
2 .M 2 .m 2. W
2. rn 2.362
0. SIB
i .s e
-
0.116
0. 1'2 0. 1%
V . 137
[I. 145
0.153 0.182 0.170 0.180 0.189
0:Mo 0.210
0.221
0.232 0.245
0.259 0. W! 0.m
0.904 0.320
0. $37 0.354 0.370 0.388 0.4K
O.&X
0.447 0.470 0.4% 0.517
0.543
0. 571
0.599
0. 6E4 0.659
(1. 690
0. F29
0. 756 0. 790 0. &E
0. &I
0.w 0.9B 0.993
1.m
1.w 1.1% l .l &
1.241
1.297
l.w 1.41: 1.471 1.53f
1.591
1.6%
1.W 1.78:
1.85:
I.*&
1.9% 2. w 2.14:
2. 2& 2. 3ot
8.3%
2.4R 2. 68; a. I( 2.7%
~
0.149 0.157
0.166
0. 176 0.1%
0.1% 0.m
0.219 0.231 0.243
0.2% 0.270 0.284 0. SI 0.315
0.33
0.350
0.370 0.391
0.41 1
0.433 0.454 0.470 0.499 0. 324
0.549 0.575 0.601 0.634
0.665
0.699
0.734 0.770
0.808
0.817
0. RA7
0.929
0.972 1.015 1.083
1.112
1.165 1.520
1.270 1.335
1.3%
1.461
1.522
1.596
1.m
I . 742
1.902 1.975 2.051
9. lee
2.!?11
2.381
2.4i1
2.561
2.6M
2. is 2.85s
2.96:
3.073 3.1% 3.m 3.4% 3.54
1 .8 ' ~
?. 29s
-
0.1M 0.174 0.1M
0.1%
0 .2 x
I). 2lt
0.31
0. !a2 0. El?
0 .m
0.'W
0.w 0.31t 0. -3%
0.3%
0.m
0.411
0.424
0.45;
0 8 1
0.501
0. $2
0.551
0.5%
0. 61( 0.635
0.671
0 .W
0.73
0. i7t 0.81(
0.8% 0.8% 0.931
0.w
1.03:
1 .0 8
1.15
1.181
1.3:
1.L2
1.411
1 .a
1.55 1.6% 1.6V
1.77: 1.85:
1.93! 2.021 2.11, 2.19 8.27'
2.361
2.45'
2.53 2.64
2.741
2.84 2.95 3.06 3.17 3.29
3.41 3.53
8.06
3.80 3.93
0.370
? .35:
-
0.1% 0.139 0.147 0.157
0.166
0.174
0.185
0. ltLi
0. m 0.216
0.240 0.253 0.266 0.250
0.'% 0.311 0.3!2 0.347 0.366
0.385
0.404
0.423 0.443 0.466
0.488 0.511 0.537
0.583
0.591
0.621
0. G53
0. (i85
0.718
0.753
0. i89 0.626
0. w 0.902
0. w5
0.989 1.035 1.w 1.134 1.186
1.241
l .L .2
1.358 1.418 1.4Y2
1.548 1.618 1.6W 1.755 1.823
1.893
1.968
2. a0 2.117
2.1%
2.279 2.364
M. 512
2.635
2.731 2.831 2.934 3.040 3.149
0 .m
%451
-~
0.W3 0.087 0.W2 0. o!A¶
0.104
0. 109
0.116
0.122 0.1%
0. 195
0 .w u.150
0.158
0.166 0.175
0.1P5
0.191
0. m 0.217
0 .9 8
0.210 0. m 0.264
0.57 0.291
0.905 0. *a 0.986
0.35'2 0.3iu
0. %f!
0. 'W
0.42f
0.449 0.450
0.409 0.516 0.5il
0.5W 0.591
0.64;
0.6i'l 0. i@
u. 742
0. 77t 0.1112
0. w 0.W 0.93
0. w 1.011 1.W
1.09;
1. I#
1 .H 1.w 1.27! 1.3% I . 37:
1.4%. 1.471
1.59:
1.w 1.64'
1.7g
1.771 1.83' 1.w
1.961
0.611
-20 19 18 17 16
-15 14 13 1'2 11
-10 9
! A
-5
4 3 2
-1
0
3 4
5
+;
I A 9
10
11
12 13 14
15
16
17
18
19
20
21
29
3 24
95
%
?i
28
I
24 31 35 3.. 31
3t
36 35 L 3:
4c 41
4:
4: 41
I t .u 4i
41
$41
0.017 0.V17
0.018 0.020
0. (PA
0.@2
0. o!! 0.va 0.028
0 .0 3
0.v3
0.030
0.032 0. m3
0.035
0.037 0.039 O.(U1 0 .M
0 .M
0.04R
0.0M
0.053
0.055
0 .M
0.061 0.061
0.06s
0. KU
0.074
0 .m 0. a:! 0. OM
0. ox 0. ow
0. of& 0. lo?
0.m 0.113 O .l l t
0.124
0.1%
I). 13f 0.1.I:
0.141
0.151
0.165
0.13 0. In 0.m
0.194 0.20:
0.211 0.2a 0.M
0.23:
0. ?'l( 0.251
0.243!
O .Z !
0.28
0.2%
0.3M 0.311
0.32
0.34 0.35 0.30' 0.38 0.39
0.033 0.035 0.037 0.039 0.041
0.044
0. (U6 0.019 0.m1
0.054
0.067 0.060 0.009 0.CM
0. KO
0. (ti4 0.078
0,082
o.oM7 0.091
0. IM 0.101 0.1% 0,111
0.116
0.122
0.18 0.134 0.141 0.148
0.155
0. I63 0.171 0.180
0.1%
0.197
0. !a6 0.210
0 .2 6 0.236
0.247 0.259
0.L71
0. w 0. L ?
0.310 0.325 0.339 0.355 0.371
0.38;
0. 'mi 0.422 0.439 0.456
0.473 0.491 0.516 0.5% 0.54s
0.57c 0.591 0.613 0.63: 0.65:
0.w 0. 7M 0.
0. $61
0.5%
0.050
0.055 0.059 0.06"
0.065 0.069 0.053
0. m 0.081
0.086
0. u90
0.095
0.100
0.111 0.117 0. I:? 0.130 0.137
0.144
0.158
0.159 0.166 0.15'6
0.189
0.192
0.201
0.211
0.22
0.239
0.245
0. %7 0. !&9
0. !B2
0 .a 0.316 0.3%
0. 0.354
0.371 0.w 0.4M 0.4%
0.44;
0 .a
u. 48: 0.w 0.m
0.5%
0.5%
0. eo; 0.68 0.6%
0.68r
0.71(
0.S&
0.76! 0.79
0.8.2'
0.m
0. W 0.91! 0.96:
0. w
l.m 1.m ].lo( 1. la 1.18
n. 052
0. 105
0.066
0. OiO
0. Oi4
0. (178
0.083
0. m7 0.092
0.W 0.103
0.108
0.114 0.120
0. 1'26 0.1% 0.140
0.148
0.156 0.164 0.174
0.193
0.192
0.202
0.21'2 0.232 0.233
0.244 0.258 0. %8 O.'B?
0. m
0.310
0. %.a 0.342 0.359 0.3%
0.394 0.413
0.432 0.451 0.4;2
0.404 0.518
0. M3 0. 567 0.593
0 .6 3 0.64Y 0. 6iY
0. 709 0.i41
0. i74 0. Hos 0. $45
0.878 0.912
0.946 0.983 1.pa 1.0s
1.w
1.14c 1 .m I.?! 1.m 1.31E
1.3W 1.41t 1.465
l.m 1.574
No~e.-The following example o f the use of the above table indi- *atea how interpolation for intermediate percentsges of Raturatioii may be efferted: What is the weight of vapor in a cubic foot corresponding to a tem- peiatur_e of 70' and a relative humidity of S3 per cent? At 40' and SO per cent the weight ia . . . . . . . . . . . . . . 6.354 grs. The additional weight fo? 3 per cent is 1/10 the weight for 30 per cent, viz.. . . . . . . . . . . . . . . . . . . . . . .0394 grs.
Hence t h e weight at 70' and 63 per cent is.. . . . . . 6.623 grs.
--
Relative h 1imidity.-In order to utilize the foregoing table
in practical work i t is necessary to determine the percentage
of humidity in any given case. Generally, i t will suffice
simply to mefisure the moisture present in the air adjacent
to the place a t which operations are being conducted, or a t
the point a t which the air is being drawn into works, kilns,
etc. One of the best instruments for this purpose is known
as the sling, or whirling, psychrometer, consisting of a pair
MAY, 1898. MONTHLY WEATHER REVIEW. 207
of thermometers, provided with a handle as shown in Fig. 1,
which permits the thermometers to be whirled rapidly, the
bulbs being thereby strongly affected by the temperature of
and moisture in the air. The bulb of the lower of the two
thermometers is covered with thin muslin, which is wet a t
the time an observation is made.
The wet bulb.--It is important that the mus-
lin covering far the wet bulb be kept in good
condition. The evaporation of the water from
the muslin always leaves in its meshes a small
quantity of solid material, which sooner or
later somewhat stiffens the niuslin so that i t
does not readily take up water. This will be
the case if the m u s h does not readily beconie
wet after being dipped in water. On this ac-
count it is desirable to use as pure water as
possible, and also to renew the muslin from
time to time. New muslin should always
be washed to remove sizing, etc., before being
used. A small rectangular piece wide enough
to go about one and one-third tinies around
the bulb, and long enough to cover the bulh
and that part of the stem below the metal
back, is cut out, thoroughly wet in clean water,
and neatly fitted around the thermometer. It
is tied first around the bulb a t the top, using
a moderately strong thread. A loop of thread
to form a knot is next placed around the Lot-
toni of the bulb, just where it begins to round
otY. As this knot is drawn tighter and tighter
the thread slips of€ the rounded end of the
bulb and neatly stretches the muslin covering
with it, a t the same time securing the latter
a t the bottom.
To make nri obserrntion.-The so-called wet
hulb is thoroughly saturated with water by
dipping i t into a small cup or wide-mouthed
bottle. The thermometers are then whirled
rapidly for fifteen or twenty seconds; stopped
and quickly rend, thewet bulb first. Thia read-
ing is kept in mind, the psychrometer imme-
diately whirled again and a second reading
taken. This is repeated three or four times,
or more, if necessary, until a t least two suc-
cessive readings of the wet bulb are found to
agree very closely, thereby showing that i t has
reached its lowest temperature. A minute or
inore is generally required to seciire the correct
temperature.
When the air temperature is near the freezing
point it very often happens that the teml~era-
ture of the wet bulb will fall several degrees
below freezing point, but the water will still
remain in the liquid state. No error results
from this, provided the minimum temperature
is reached. If, however, as frequently happens,
the water suddenly freezes, a large amount of
FIG. l.--Sling heat is liberated, and the temperature of the
psychrometer- wet bulb immediately becomes 3 2 O . I n such
cases i t is necessary to continiie the whirling until the ice-
covered bulb has reached a minimum temperature.
Whirling and stopping the psychroneter.-It is impossible to
effectually describe these movements. The arm is held with
the forearm about horizontal, and the hand well in front. A
peculiar swing starts the thermometers whirling, and after-
ward the motion is kept up by only n slight but very regular
action of the wrist, in harmony with the whirliiig ther-
mometers. The rate should be a natural one, so as to be
easily and regularly maintained. If too fast, or irregular,
the thermometers may be jerked about in a violent and dan-
4eroue manner.
The stopping of the psychrometer, even a t the very highest
rates, can be perfectly accomplished in a single revolution,
when one has learned the knack. This is only acquired
by practice, and consists of a quick swing of the forearm by
which the hand also describes a circular path, and, as i t were,
follows after the thermometers in a peculiar manlier that
wholly overcomes their circular motion without the slightest
shock or jerk. The therinometers may, without very great
danger, be allowed simply to stop themselves ; the final mo-
tion in such a case will generally be quite jerky, but, unless
the instrument is allowed to fall on the arm, or strikes some
object, no injury should result.
E.cposure.-W hile the psychrometer will give quite accu-
rate indications, even in the bright sunshine, yet observations
so made are not without some error, and, where greater ac-
curacy is desired, the psychronieter should be whirled in the
shade of a building or tree, or, as may sometimee be nece8-
sary, under an umbrella. I n all cases there should be per-
fectly free circulation of the air, and the observer should
face the wind, whirling the psychrometer in front of his
body. It is a good plan, while whirling, to step back and
forth a few steps to further prevent the presence of the nb-
server’s body from giving rise to erroneous observations.
The relation between the readings of the psychrometer
and the pressure of the vapor of water mixed with the air is
not perfectly understood, a1 though several empirical for-
n i u l ~ have been developed which express this relation more
or leas exactly. The tables employed by the Weather Bureau
were conipiited by Professor Ferrel’s formula, the constants
of which were deterniinecl from a large number of compara-
tive observations of the psychrometer and Regnault’s dew-
point apparatus (see W. B. No. 137). The formula is:
$1 = F - O.OOO360 (t -i ’ ) ( l+O.O0065 t ’ ) P
p is the desired pressure of the aqueous vapor.
F is the niaximiiin pressure corresponding to saturation a t
t equals the air temperature ; t’ the wet bulb temperature,
the temperature of the wet bulb.
and P the barometric pressure.
THE UMBRELLA CLOUD.
By Mr. WILLARD D. JOENSON.
In the Meteorologische Zeitschrift for January, 1896, M.
Streit has given an illustration of a remarkable cloud forma-
t,ion, designated as “ umbrella cloud,” observed in northern
Italy. Recently the Editor became aware of an equally inter-
esting forniation carefully observed in Kansas and also called
an ‘‘umbrella cloud” by its discoverer, Mr. Willard D. Johnson,
of the U. S. Geological Survey. A h . Johnson made t w u
sketches of the cloud in his field notebooks and subsequently
hJr. DeLancey W. Gill made a more elaborate drawing for
him. Reprints of these, by photogravure, are given in the
acconipanying charts, SI and X I . The Editor deems it
iniportant to reproduce the sketches from the field notes, in
order that the student may distinguish between those fea-
tures of the completed drawing that have been filled in from
memory and those that have the sketches as a basis. Wr.
Johnson writes as follows under date of May 13, 1898:
My point of view was 1 mile northwest
of Garden City, Kans. The time was about ten minutes of 4 p. m.
[? central time]. I was looking nearly due west. The cloud was also
observed by MI.. H. W. Menke, of Garden City, a graduate of the University of Kansas. He was about 4 miles to the northwest of my position. He made a hotograph with a sniall pocket camera. As he was not looking towarkan illuminated portion of the sky, a8 I was, the outlines were not so clearly defined. At any rate, his little photograph gives no details; the general outline, however, of the lower truncated cone is plainly distinguishable and agrees very well with the extra- ordinary form in my sketch.
The date was July 25, 1896.