SELF-PROPELLING ENGINE. 307 



nothing but a little ugly monster with a great head and staring 

 eyes, a reptile- shaped body, six hairy legs, and a sort of triple 

 pointed tail."* True, but let us watch its progress through the 

 water, with which, as we shall find presently, those six legs 

 have infinitely less concern than the tail, that triple, or, more 

 correctly, quintuple pointed tail of which you spoke. Now 

 this anal apparatus forms, in fact, the mouth of an internal 

 pump or pistorn, made instrumental to the motion of the body 

 which contains it. See how these little fragments of paper 

 which we throw into the water are alternately drawn towards 

 and repelled from the instrument in question ; and drawn in, in 

 like manner, is a portion of the water, which, regularly imbibed 

 and rejected by the insect's pump, forms a jet which propels the 

 creature through resistance of the mass of fluid in its rear. 



To show more clearly the working of this curious engine, 

 suppose, in accordance with a plan suggested by Mr. Rennie, 

 that we let down carefully, by a glass tube, over the mouth of 

 the pump in other words, over the insect's tail a coloured 

 solution. Ah ! we can see clearly, now, how he ejects it in a 

 stream which marks for several inches his wake upon the 

 water. Here then we have that pattern for the propulsion of 

 a vessel of which Paley thus speaks when writing, in 1802, of 

 discoveries or projects in imitation of Nature. 



" Some years ago/' he says, " a plan was suggested of pro- 

 ducing propulsion by reaction, this way. A. stream of water 

 * See Vignette to " A Summer Day's Dream." 



