IO                                  FLYING   MACHINES.
ity, but there will be friction of air upon them, and there
must be a solid body or hull to contain the machinery
and the cargo.
Thus the elements of resistance are three in num-
ber :
1.  The hull resistance.
2.  The drift.
3.  The skin friction.
Of the skin friction Professor Langley says that it is ap-
parently so small that it may be neglected without mate-
rial error ; and he has given the measure of the " drift"
as the result of his experiments.
The head or hull resistance will probably be found to be
the chief element which will limit the possible speed of
flying machines. It will probably grow as the square of
the velocity, thus requiring the power exerted to vary as
the cube of the speed, but will be modified by a series of
coefficients, due to the shape of the solid body, just as
some birds are swifter flyers than others of the same
weight, in consequence of their difference in shape.
Hence the power required to drive such a machine can
only be approximated at present ; but this will be more
particularly discussed when treating of the areas of sup-
porting surfaces and speed of birds, for the reader may be
impatient to be told something of what has been attempted
by man.
Inventors, in their ignorance of the laws of air reactions
and resistances, have proposed all sorts of devices for
compassing artificial flight and experimented wilh not a
few ; so that Mr. £. Dieuaide, of Paris, upon making a
study of the subject, published in 1880 an illustrated
chart,* in which he delineated the more remarkable ma-
chines which had been proposed for aerial navigation with-
out the use of balloons. This chart contains some 53
figures ; and from this, as well as from the book of M.
Gaston Tissandier on Aerial Navigation,t which contains
much accurate information, the following has been chiefly
compiled, in which it will be attempted not only to give an
account of what has been proposed, so far as the meager
data will permit, but also to critcise the machines with the
light of our present knowledge, and to endeavor to point
out why they failed. Failures, it is said, are more in-
structive than successes ; and thus far in flying machines
there have been nothing but failures.
These various machines, diverse as they are, may rough-
* Tableau d'A viation. Représentant tout ce qui a été fait de remarquable
sur la navigation Aérienne sans Ballons.   Published by the author.
t La Navigation Aérienne. Par Gaston'Tissandier. Hachette et Cie ;
octavo, 334 pp.