PHENOMENA OF FLIGHT IN THE ANIMAL KINGDOM. 253 



be rationally explained, if we recollect that tlie rubber becomes heated 

 when stretched. We may then naturally suppose that the heat vrhich 

 appears upon its surface will disappear by degrees if the traction be 

 prolonged ; and if heat is necessary for the contraction of the rubber to 

 its original dimensions, it will not contract entirely if a proportion of 

 the heat disengaged in stretching has been dissipated. If this theory 

 be correct, it will be easy to rapidly produce a passive condition of the 

 rubber by quickly depriving it of its sensible heat. This is precisely 

 what takes place. Stretch a thread of rubber, and plunge it into cold 

 Avater, and it can instantly be taken out in a passive condition, frozen, 

 so to speak, in a state of elongation. Heat it, and it returns to its 

 original dimensions with the development of power. I hardly doubt 

 that the physicist can obtain in the power thus developed the exact 

 equivalent of the heat absorbed. 



We have strayed away from our subject, but we shall return to it with 

 enlarged ideas which will enable us to analyze more completely the ac- 

 tion of the nniscles. In fact, we have in the employment of the percha 

 a sort of schema of the muscle. Kow you are aware of what assistance 

 can be derived from theoretical apparatus in the study of certain phe- 

 nomena which are presented vrith too much complexity, in the case 

 of living beings, to be understood readily. The rubber will assist us 

 to comprehend the manner in which the wofk of the muscles is per- 

 formed in animals in general, and especially in the birds with vrhich we 

 are now occupied. Take two cylinders of rubber of the same dimen- 

 sions and weight, stretch them to ten times their original length, and 

 cool them in that condition. If we restore to these two cylindrical 

 threads the amount of heat which they h:id lost, both in contracting 

 will perform the same amount of work in a similar manner ; that is, 

 both will lift the same weight to the same height. Next, taking two 

 cylinders of the same weight, but of unequal diameter, one, let us sup- 

 pose, being ten times as thick but only one-tenth as long as the other; 

 stretch each ten times its normal length and cool it; both will still be 

 able to do the same amount of work, but no longer in the same manner. 

 The short, thick cylinder, for instance, will raise a weight of 100 grams 

 a distance of one centimeter. The long, thin cylinder will not support 

 so great a weight at all, but if loaded with a weight of 10 grams will 

 raise it a distance of 10 centimeters. The measure of a mechanical 

 power is obtained bj^ multiplying the weight raised by the height 

 through which it is raised. This product being the same in the two 

 cases, the same amount of power, therefore, has been expended, but not 

 in the same way. So, in the cylinders of rubber which have been 

 stretched in proportion to their length, and from which the same amount 

 of heat has been withdrawn, the amount of ])Ower produced by the res- 

 titution of this heat will be proportioned to the weight of the rubber ; 

 the weight lifted to the diameter of the rubber, and the height to which 

 the weight will be lifted, to the length of the rubber. All that weknow 

 of the function of muscle tends to prove that the power exhibited 

 by it is governed by the same laws. Thus, the amount of contraction 

 of muscles depends on the length of their fibers, while the maximum 

 power which they are capable of exerting is proportional to the diame- 

 ter of the bundle of muscular fibers. For example, in the case of hu- 

 man muscles, the short, thick deltoid muscle is capable of but little 

 contraction, but exerts great force ; the long, thin muscles of the fore- 

 arm, on the contrary, cannot develop the same power, but by their os- 

 seous attachments the extent of their contraction is much greater. If 

 we sui)pose these two muscles to be of the same weight, they can do an 



