yan. 22, 1874J 



NATURE 



221 



and this would enable us far better to understand how the 

 tropical forms of life continued to flourish by migrating 

 north or south away from the colder pole. The subject 

 of glacial periods is rendered vastly more difficult by the 

 discovery of signs of glaciation so far within the tropics, 

 rnd all facts proving such glaciation are of the greatest im- 

 portance. It seems most probable that the solution of 

 the problem will be only possible by admitting a succes- 

 sion of glacial periods of unequal intensity ; so that while 

 in the tropics we have the traces of one of the more an- 

 cient and intense period of cold, in the more northern 

 regions we see the results of successive glaciations and 

 intervening denudations. 



Much more satisfactory as well as more original, is Mr. 

 Belt's theory of the cause of whirlwinds and cyclones. 

 He well remarks that there is a complete gradation, from 

 the little eddy which whirls up the dry leaves, through the 

 moderate whirlwind, up to the most destructive hurricane ; 

 and that a great philosophical mistake has been com- 

 mitted in forming theories to explain the larger pheno- 

 mena without ever having studied the smaller. The few 

 pages devoted to this subject are well worth reading, and 

 would alone stamp the author as an acute observer in 

 physics as well as in natural history. He gives good 

 reasons why all the received theories of the cause of 

 cyclones are incorrect, and substitutes one founded on ob- 

 servation of the smaller and more easily observed pheno- 

 mena which is very ingenious, and which appears to have 

 received the provisional approval of the Astronomer 

 Royal, but which would occupy too much space to give 

 an account of here. 



We have now sufficiently shown that most of the readers 

 of Nature will find matter of interest in this volume ; 

 and we sincerely trust that the author may soon find him- 

 self in a position to work more systematically at some of 

 those branches of science which he has here touched 

 upon. So clear-sighted and intelligent a student will pro- 

 bably make important discoveries. 



Alfred R. Wallace 



PETTIGREW'S ANIMAL LOCOMOTION 

 Animal Locomotion ; or, Walking, Swimming, and Fly- 

 ing. By J. Bell Pettigrew, M.D., F.R.S. (London : 

 Henry S. King and Co., 1873) 



PROGRESSION on land, in water, and in air, are 

 phenomena so intimately connected with everyday 

 life, that all of a thoughtful and observant turn of mind 

 cannot help becoming acquainted, unassisted, with most 

 of the details and much of the principle of their produc- 

 tion. Many will therefore open a new work on the sub- 

 ject with a wish to have explained to them some of the 

 more difficult and obscure problems connected with it, 

 which are too intricate or uncommon to be within the 

 limits of ordinary powers of observation ; and to have 

 the fundamental principles on which the subject is based, 

 fully expounded. With such a feeling we took up the 

 book under consideration, especially as Dr. Pettigrew's 

 name has been always held up as that of the British ex- 

 ponent of the phenomena of flight, and the combatant of 

 the French school. Imagine our disappointment on find- 

 ing that, instead of the work being by the hand of a 

 master, its author is deficient in the knowledge of the 



first principles of physics, and of the undoubted me anin g 

 of some of the most simple terms employed in the sci- 

 ence ; his argument, if it may be so called, being but 

 little more than a long series of vague and fanciful ana- 

 logies, incorrectly stated physical facts, and untenable 

 theories. 



In the introduction, and more minutely in a special chap- 

 ter, the subject of aeronautics is discussed, and the false 

 hope perpetuated that it is quite within the range of human 

 possibility to construct a flying machine, capable of sus- 

 tained suspension ; for we are told that " in order to 

 construct a successful flying machine ... all that is 

 required is to distinguish the properties, form, extent, 

 and manner of application of the several flying surfaces ; " 

 no mention being made of the true difficulty of the 

 problem, which is, that it is at present impossible to 

 obtain from any form of fuel, a sufficient percentage of 

 the potentiality which it possesses for doing work, to 

 work an engine sufficiently compact and light for the 

 wings which it has to drive. In the chapter on progres- 

 sion through the air, one of the paragraphs commences 

 with the astonishing title, " Weight, Momentum, and 

 Power, to a certain extent synonymous in flight," which 

 follows an equally extraordinary and oft-repeated state- 

 ment that " weight, when acting upon wings, or what is 

 the same thing, upon elastic twisted inclined planes, must 

 be regarded as an independent moving power." After 

 such indications of imperfect knowledge, nothing in the 

 way of mechanical theories could cause surprise, and we 

 are therefore not astonished to find it laid down as the 

 fundamental principle of flight, that the up-stroke of the 

 wing aids in propulsion, and that in the down-stroke the 

 inferior surface of the wing is directed downwards and 

 forwards. " I repeat downwards and forwards ; for a 

 careful examination of the relations of the wing in the 

 dead bird, and a close observation of its action in the 

 living one, supplemented by a large number of experi- 

 ments with natural and artificial wings, have fully con- 

 vinced me that the stroke is invariably delivered in this 

 direction," the wings being said to act like a boy's kite 

 during both ,the down and up stroke. Who can see any 

 close relation between the flight of birds and that of a 

 kite ? Dr. Pettigrew seems to forget that a kite needs a 

 string, and yet, backed by his false analogy, he has the 

 presumption to quote the experimental verifications and 

 opinions of such able and ingenious thinkers as Borelli 

 and Marey, the authors of the true theory of flight, only to 

 reject them ; bringing forward in opposition such evidence 

 as " from accurate examination, I am fully convinced," 

 and the like, against the sound mathematical arguments 

 and superbly conducted experiments of the two above- 

 named physicists. 



Another favourite notion which Dr. Pettigrew re- 

 iterates is that " the efficiency of the wings is greatly 

 increased by the fact that when it ascends it draws 

 a current of air up after it, which current, being met 

 by the wing durmg its descent, greatly augments the 

 power of the down-stroke. In like manner, when the 

 wing descends, it draws a current of air down after it, 

 which, being met by the wing during its ascent, greatly 

 augments the power of the up-stroke. . . . The wing is 

 endowed with this remarkable property, that it creates the 

 currents on which it rises and progresses." This would 



