368 
truth of the kinetic theory. When a colloid 
suspension in water is observed with an ultra- 
microscope, the number of particles in the field of 
view is constantly fluctuating, because the particles 
are moving haphazard. By observing the extent 
of the fluctuations, the author has been able to 
test how far the behaviour of the colloid is in 
agreement with the simple gas laws. He has 
also invented a most ingenious method of measur- 
ing the concentration fluctuations in a molecular 
solution. A solution of a polonium salt in water 
‘is used, and the rate at which a particles are 
shot off provides a measure of the con- 
centration of the solution. Unfortunately 
radio-active change is just as haphazard 
as the movements of molecules, so that the 
fluctuations in the emission of a@ particles are due 
to the two causes combined. Experimental difh- 
culties have been overcome, and the two effects 
separated, the result being in agreement with the 
kinetic theory. 
This book is not merely a collection of reprints, 
for the original papers have been re-written into 
a connected whole, while some of the material had 
not previously been published. | Unfortunately, 
however, the author has made no attempt at con- 
densation, and the very interesting subject-matter 
is at times lost in a plethora of numerical results. 
Considerable tracts, such as pp. 152-164 and 
183-195, resemble a laboratory note-book, and 
even the lucid descriptions of experimental 
arrangements leave too little to the reader’s 
intelligence. In a smaller book the importance 
and ingenuity of the experimental work would 
have held the reader’s interest. H. G. J: ME 
FLIGHT PRINCIPLES AND PRACTICE. 
(1) The Flight of Birds. By F..W. Headley. 
Pp. x+163+xvi plates in text. (London: 
Witherby and Co., 1912.) Price 5s. net. 
(2) The Mechanics of the Aéroplane. A Study of 
the Principles of Flight. By Capt. Duchéne. 
Translated from the French by J. H. Ledeboer 
and T. O’B. Hubbard. Pp. x+231. (London: 
Longmans, Green and Co., 1912.) Price 7s. 6d. 
net. 
(1) R. HEADLEY is well known, in the 
Aéronautical Society and. outside it, for 
his interesting studies of bird flight. If he has 
not much that is novel to say about the more 
controversial points on which some difference of 
opinion still remains, he is yet able to write very 
pleasantly and instructively, and often from his 
own observation, about all the main features of 
bird flight; and the series of the author’s photo- 
graphs selected for reproduction makes one wish 
for more. 
NO. 2300, VOL. 921 
NATURE 
[| NOVEMBER 27, 1913 
The point of view throughout is that of the 
interested and intelligent observer, describing and 
so far as possible explainifg his observations. — 
The explanations are given in the simplest pos- 
sible manner, and are as clear as need be. The 
opening chapter on gliding flight might perhaps” 
be improved in the light of the most recent know- 
ledge, but the criticism is of no importance; the 
book is written for the lover of nature, not the 
technical expert. The questions dealt with include 
stability and steering, starting and alighting, — 
soaring, pace and endurance, varieties of wing 
and of flight, flight machinery and some accessory 
characteristics. It is a volume which will be read 
with pleasure by those interested in flight, and in 
birds. ; 
(2) The purpose of this book, in the words of 
the translators’ preface, is “to explain in terms 
as simple as possible, and with a minimum ‘of — 
formule, the main principles of dynamic flight; 
to give the ordinary reader an insight into the 
various. problems involved in the motion and 
equilibrium of the aéroplane; and to enable him ~ 
to calculate in the simplest possible manner the 
various elements and. conditidns of flight.” 
Judged from this point of view, the book is one 
which can be cordially commended to the “ordinary — 
reader,” for whom it is written. The elementary 
principles of aérodynamics applicable to the aéro- 
plane as expressed by the usual formule derived 
from experimental observation, are given in their 
simplest, approximate, form; and the logical 
deductions which can be made from these formula 
are explained with care and conciseness. All the 
main points of importance can be brought out 
in this manner, and the result will enable the 
reader who is prepared to take the small amount 
of trouble required to follow the argument to 
obtain an intelligent grasp of the conditions under 
which the flight of an aéroplane can be sustained. 
The reader new to, the subject will probably be 
surprised to find how simple and how few in 
number are the fundamental formule and ideas 
required. These comprise the formule expressing 
the “lift” and “drift” in terms of the velocity 
and angle of incidence; a little information as to 
the movement of the centre of pressure; and some 
general ideas as to the effect of camber, and of 
aspect ratio. With this apparently slender equip- 
ment, and some acquaintance with the laws of 
elementary mechanics, a tolerably complete dis- 
cussion is given of the main principles of flight, 
including even a chapter on the screw-propeller. _ 
The apparent simplicity thus obtained is, 
indeed, from another point of view, one of the 
main defects of the book. The simple law 
assumed for the variation of lift with angle of 
