esa The Mechanics of Soaring. [June, 
I was unable to secure any photographs, though a friend kindly 
tried to take some for me, our ladder proving too short to admit 
ie of getting the camera into position for focussing ; but the rough 
4 drawing made on the spot with the aid of an opera-glass may 
give a better idea of the feeding position assumed by the mother . 
bird than my description has done. 
10; 
THE MECHANICS OF SOARING. 
BY PROFESSOR J. E. HENDRICKS., 
; A Mr. I. Lancaster has published, through the medium of the 
American Naroratist, his very interesting and valuable 
observations of soaring birds, and has, in the April number (No. 
4, Vol. xx) given an explanation of the mechanics of soaring that 
might lead non-technical readers astray, a brief review of the 
“ Mechanics of Soaring ” may not be unprofitable to some of the 
readers of the NATURALIST. ; 
<< As much that Mr. Lancaster has said is in accordance with 
the recognized principles of mechanics, I will not encumber 
the pages of the NATURALIST with a general review of the whole 
article, but will confine this paper mainly to a consideration of the 
question proposed by him and which he regards as a crucial 
_ test of the validity of his theory of soaring. al 
In investigations concerning the operation of forces, it 1s 1M- 
s portant that the distinction between continuous and momentary 
ee forces be kept in view. Although all forces require time for their 
operation, yet such forces as act for a short time and then eeu 
to act, are called momentary forces, and the time during which 
they act is not considered ; the velocity induced being constant 
_ and equal to the intensity of the force divided by the mass. t 
= When a force acts uniformly for a considerable portion © 
time, it is called a constant force, and the time of its action ee 
_ involved in the velocity it induces, which is represented by the 
__ intensity of the force multiplied by the time and divided by the 
mass or weight of the body. : 
_ Although we do not know what produces the ROE. 
_ of gravitation, we know, as manifested on the surface* “ d 
earth, it is a result of two opposing forces (a centripetal peas 
~ centrifugal force) whose difference at any point on the earth 
S 
a 
