386 



BORELLI ON SWIMMING. 



Now, suppose the fish is a machine similar to the syringe, CABM, and 

 that such fish contains in its abdomen a small bag, e, filled with air, 

 which, in a moderate constitution, may be compressed in such a manner 

 as to render the bulk of the fish equal to the bulk of the equally 

 pendent water ; in that case the fish can remain without motion im- 

 mersed in the water. But if it wishes to become lighter, in order that 

 its machine may rise to the surface spontaneously, it is necessary, that 

 by relaxing the abdominal muscles, it can do so, and that the air, by 

 exercising its elastic power, can expand into a larger space, d ; and, on 

 the contrary, if it wishes to be of a greater specific weight, and to sink 

 spontaneously to the bottom, it is necessary, that by a contraction of 

 the muscles of the abdomen, it tightens the air-bag, like a press, 

 until it occupies a small space, f. 



Experience proves, that boats impelled by a single oar placed at the 



