i877-] 
Notices of Books 
in 
A man in walking a mile has done work, as far as his legs are 
concerned, equal to that which would be required to raise him 
vertically through about 146 yards ; and if we reckon that a 
sound man can walk 30 miles a day, he would expend sufficient 
force to raise himself and an apparatus of wings weighing 
10 lbs. through a vertical height of 2^ miles. If we suppose 
that his wings have each an area of 7 square feet, they must 
move through 18 feet in a second in order to neutralise the 
weight of the system, and any power above this could be used 
for raising and propelling the body. To remain poised in the 
air the wings must be kept vibrating with such a velocity that 
they move through 25^ feet per second. We may suppose a 
man, without overtaxing his strength, to be able to make ten 
complete beats of his wings in a minute. The wings would 
have to be very gigantic : according to a calculation given on 
page 30 the distance to the centre of the wing must be 153 feet. 
The first essential condition of aerial navigation is stated to be— - 
“ That the propelling power must be so adjusted that it may 
be kept constantly horizontal, and that the force must be so 
applied that it shall have no tendency to make either gas-vessel 
or man-vessel deviate from a strictly horizontal position, and this 
whatever may be the speed of flight.” — (P. 51.) The second 
condition, is — “ That the gas vessel must be kept perfectly stiff, 
so that it shall neither turn up nor be bent down at the ends, but 
shall keep its form under all varying conditions of load from 
below and of gas-charge within.”- — (P. 55.) 
Many proposals have been made as to the shape of the gas- 
vessel. Sir George Cayley, in 1816, proposed that it should 
have the shape of a woodcock’s body ; others have proposed 
prolate spheroids and spheres ; others a fish-like form. Our 
author concludes his chapter on the question of shape by the 
assertion— “ The vessels of air-craft must be of an elongated 
form, to enable them, by cleaving the air and eluding its resist- 
ance, to receive the highest velocity attainable by the exertion 
of a given amount of power.” The material must be imperme- 
able to gases, and at the same time strong and light, and it must 
be charged with the lightest possible gas. Hence a cheap way 
of preparing large quantities of hydrogen must be found. The 
air-craft must retain its buoyancy unaltered : it must be able to 
retain both gas-vessel and man-vessel in a horizontal position, 
and to regain its level if disturbed. Then comes the great 
question of power. Numberless suggestions have been made on 
this score : steam, electro-magnetism, human muscular power, 
the tradtion of flying birds, &c. Then there must be appliances 
for transmitting to the outside atmosphere the force generated 
within the balloon, in such a way that there will be sufficient 
leverage to propel the craft forward. An interesting summary of 
the requisite conditions is given in the last chapter of the first 
part (pp. 174 to 182). 
