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ISTATURE 



fFeb. i^, 1888 



capable of transporting man through the air, but many 

 of them are troubled by a doubt ; for they ask themselves 

 whether the force of the bird does not exceed that of the 

 known motors. The experiments on that subject may 

 reassure them, for, if we compare the muscular force of 

 the bird with that of steam, we see that one muscle would 

 be comparable to an engine at very low pressure. In 

 fact, the steam which would develop 1*600 kilogramme 

 per square centimetre would scarcely have more than an 

 atmosphere and a half of pressure. But the true com- 

 parison to establish between the animated motors and 

 the engines consists in measuring the work which each 

 of these motors can furnish, with equal weight, in the 

 unity of time. 



The measure of the work of a motor is obtained by 

 multiplying the effort put forth, by the path which the 

 point of application of that effort traverses. Photo- 

 chronography expresses at each moment the spaces tra- 

 versed by the mass of the bird and the displacement of 

 the centre of pressure of its wings, giving thus the factor 

 path in the measure of the work. In this way it is found 

 that for the five strokes of its wing which the sea-gull 

 gives every second, at the moment when it flies away, 

 the labour done would be 3-668 kilogrammes. This calcula- 

 tion is very high ; it corresponds to that which an engine 

 would make in raising its own weight to a height of more 

 than 5 metres in a second. 



But that is only a maximum which the bird does not 

 attain to except at the moment of flight, when it has not 

 attained much speed. In fact, according as the passage 

 .of the bird is accelerated, the air under its wings presents 

 a more resisting fulcrum. I have previously experiment- 

 ally demonstrated this fact, announced by the brothers 

 Planavergue, of Marseilles, and of which the following is 

 the theory. 



When the bird is not yet in motion, the air which is 

 struck by its wings presents, in the first instance, a 

 resistance due to its inertia, then enters into motion, and 

 flies below the wing without furnishing to it any support. 

 "When the bird is at full speed, on the contrary, its wing 

 is supported each moment upon new columns of air, each 

 one of which offers to it the initial resistance due to its 

 inertia. The sum of these resistances presents to the 

 wing a much firmer basis. One might compare a flying 

 bird to a pedestrian who makes great efforts to walk on 

 shifting sand, and who, in proportion as he advances, 

 finds a soil by degrees firmer, so that he progresses more 

 swiftly and with less fatigue. The increase of the resist- 

 ance of the air diminishes the expenditure of labour ; the 

 strokes of the bird's wing become, in fact, less frequent and 

 less extended. In calm air, a sea-gull which has reached 

 its swiftest, expends scarcely the fifth of the labour which 

 it had to put forth at the beginning of its flight. The 

 bird which flies against the wind finds itself in still more 

 favourable conditions, since the masses of air, continually 

 renewing themselves, bring under his wings their resist- 

 ance of inertia. It is, then, the start which forms the 

 most laborious phase of the flight. It has long been 

 observed that birds employ all kinds of artifices in 

 order to acquire speed prior to flapping their wings : some 

 run on the ground before darting into the air, or dart 

 rapidly in the direction they wish to take in flying ; others 

 let themselves fall from a height with extended wings, 

 and glide in the air with accelerated speed before flapping 

 their wings ; all turn their bill to the wind at the moment 

 of starting. 



My experiments have, up to the present, only been able 

 to apply to the flight of departure. In order to study the 

 full flight there are conditions difficult to realize. With a 

 courtesy for which I thank him, M. Eiffel has offered to 

 me on the gigantic tower which he is erecting (at Paris) a 

 post of observation which will leave nothing to be desired. 

 From that enormous height, birds photographed during a 

 long flight will give photochronographic images much 



more instructive than those which I have hitherto been 

 able to obtain. 



Without entering into the dry details of experiments 

 and calculations made,^ I have aimed at showing that the 

 movements of birds, if they escape the sight, may be faith- 

 fully recorded by a new method which is applicable to the 

 most varied problems of rotation and of mechanics. 



Photochronography, in fact, gives experimentally the 

 solution of problems often very difficult to solve by 

 calculation. 



Imagine a certain number of forces acting in different 

 ways upon a known mass ; the complicated way in which 

 those forces are arranged sometimes renders long calcula- 

 tions needful in order to determine the positions which the 

 moving object will occupy at successive moments ; whilst 

 if the body itself, submitted to those different forces, can 

 be placed before the photochronographic apparatus, the 

 path which it will follow expresses itself upon the sensitive 

 plate. 



Distinguished physicists disputed lately as to the form 

 the free extremity of a vibrating stalk ought to present in 

 which are produced curves and nodes : the greater number 

 of them supposed that between the last node and its free 

 extremity the stalk would present a bent form. Experi- 

 ment has shown that it is not so, and that the last elements 

 of the vibrating stalk are perfectly rectilinear (Fig. 1 1), 



How many problems whose solution has formerly cost 

 efforts of genius might be solved by a very simple experi- 

 ment ! Galileo in our day would not have needed to 

 lessen the speed of the falling body in order to observe 

 its motion. He would let fall a brilliant ball before a dark 

 field, and would receive from it photographically successive 

 images. Upon the sensitive plate he would have read, in 

 the simplest way possible, the laws of space, of the speed 

 and the accelerations which he has had the glory to 

 discover. 



To return to our subject, the laws of the resistance of the 

 air to the living creatures of different forms which move 

 in it ought to be searched into by photochronography. 

 Already interesting results have been acquired : we have 

 been able to determine the path of motion and the speed 

 of small polished bodies {petits appareils planeurs) 

 which move freely in the air, and which the eye has not 

 time to follow in their rapid motions. Studies like 

 these, undertaken and methodically carried out, will 

 certainly lead to a comprehension of the still obscure 

 mechanism of the hovering of birds. 



TECHNICAL EDUCATION. 



WHEN the time comes for the discussion of the 

 "new Technical Instruction Bill, attention will no 

 doubt be given to an important series of resolutions 

 (printed on the next page) which have just been 

 passed by the Executive Committee of the North 

 of England Branch of the National Association for 

 the Promotion of Technical Education. The first six 

 of these resolutions were unanimously adopted by the 

 Committee, and the seventh was, on the motion of Mr. 

 T. Burt, M.P., seconded by Mr. J. H. Girhng (President 

 of the Trades Council), adopted with one dissentient. 

 The following are the advantages which the Committee 

 desire to secure : — (i) For primary and secondary educa- 

 tion a greater freedom of instruction under the existing 

 code preparatory to technical education in the higher 

 schools. (2) A direct or indirect pecuniary aid for superior 

 education in science and art schools and in Colleges 

 which afford technical education. (3) For all apprentice- 

 ship schools or trade classes a supervision by members of 

 the trade, but no Government grant, thus to avoid any 

 objections which might be raised by Trades Unions, or 

 any jealousy arising from an apparent protection of one 

 * See the Comptes rettdus of the Academie des Sciences 1886-37. 



