274 PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 



lateral motions to these levers. These motions are transmitted by a 

 special air-tube, as before. In the apparatus thus constructed, if we 

 move the end of one of the levers with the linger, the other lever will 

 be seen to execute the same movements with perfect fidelity. The only 

 difference consists in a slight diminution of amplitude. This happens 

 because the air contained in the tubes and drums is slightly compressed, 

 and in consequence does not transmit the whole of the motion which it 

 receives. It is easy to remedy this defect, if it be one, by placing the 

 ball-and-socket joint a little nearer the point whence the motion is 

 transmitted to the second lever. But it is better not to attem])t too 

 great amplification, because the friction is thus augmented and the 

 force which should overcome it is diminished. 



After having determined that the transmission of such motion can be 

 effected in a satisfactory manner by means of this apparatus, I have 

 sought for the means of tracing these movements upon a plain surface. 

 The difficulty which before presented itself, when I endeavored to ai)ply 

 the graphic method to the study of the wing-strokes of insects, again 

 appeared, but this time there was no means of eluding it, and 1 con- 

 tented myself with partial tracings. The point of the second lever de- 

 scribed a spherical figure in space which could not be tangent, except as 

 a point, to the smoked surface, which should receive the trace. In conse- 

 quence, I should have to register the projection of this figure on the 

 plane. Helmholtz had also encountered the same difficulty in the con- 

 struction of his myograph, and had solved it by causing the point of 

 the writing style to rub continually on the smoked surface b}^ means of 

 a weight. But as I could not attach a weight to the extremity of my 

 lever, I resolved to the following expedient, shown at the end of the 

 lever, in Fig. 24. It is large at 



the base in order to resist all ^^^' ^^ 



lateral deviations from friction; 

 this base is fixed on a vertical 

 piece of aluminium which is 

 attached to the extremity of 

 the lever. In this way the 



point of the contrivance, which ....--- 



performs the office of a style, is 

 situated exactly opposite the 

 end of the lever whose motions 

 it registers. If the lever be 

 elevated and takes the position 

 indicated by the dotted lines in Fig. 24, in traversing this space it has 

 described the arc of a circle, and its ^xtremity will be no longer on the 

 same plane as before, but the elasticity of the contrivance will have 

 carried the point of the style forward, and it will therefore continue to 

 be in contact with the plane upon which it is tracing. Thus the lever 

 elongates or shortens according as the case requires, and its point con- 

 tinually rubs upon the plane. I should add that the surface upon which 

 the tracings are received is of finely polished glass, and that the con- 

 trivance which I have used is so delicate that the pressure which it 

 exercises produces scarcely any friction. 



The apparatus being thus constructed, it must be submitted to verifi- 

 cation, to ascertain whether the motions are faithfully transmitted and 

 registered. To do this both levers of Fig. 23 are furnished with simi- 

 lar styles, placed against the same smoked glass ; and moving one of 

 the lev^ers with the hand, for instance, so as to write my name, the 

 other lever should reproduce the same signature. It frequently hap- 



Elastic point tracing upon smoked glass. 



