THE WINDMILL AND AERIAL TOP. 21 



mechanical powers as shown in Nature. The uppermost figure 

 represents a fish as in the act of swimming, the dotted lines 

 showing the movement of its tail, and the principle of the 

 wedge. In the middle is an otter, just preparing to enter the 

 water, and below is a seal, both of them showing the identity 

 of mechanism between themselves and the art of man. I need 

 not say that the mechanism of art is only a feeble copy of that 

 of nature, but nothing more could be expected. 



While we are on this subject I may as well mention two 

 more applications of the screw principle. The first is the 

 windmill, the sails of which are constructed on exactly the 

 same principle as the blades of the nautical screw. Only, as 

 they are pressed by the wind, and the mill cannot move, they 

 are forced to revolve by the pressure of the wind, just as the 

 screw of a steamer revolves when the vessel is being towed, 

 and the screw left at liberty. 



Moreover, just as the modern screws have only two blades, 

 so, many modern windmills have only two sails, the expense 

 and friction being lessened, and the power not injured. 



Again : some years ago there was a very fashionable toy 

 called the aerial top. It was practically nothing but a windmill 

 in miniature, rapidly turned by a string, after the manner of a 

 humming-top. The edges of the sails being turned down- 

 wards, the instrument naturally screwed itself into the air to a 

 height equivalent to the velocity of the motion. 



A similar idea has been mooted with regard to the guidance 

 of balloons, or even to aerial voyaging without the assistance of 

 gas, but at present the weight of the needful machinery has 

 proved to be in excess of the required lifting power. 



In fine, the application of the inclined plane, wedge, or 

 screw as a motive power, is so wide a subject that I must, with 

 much reluctance, close it with these few and obvious examples. 



It is worth while, by the way, to remark how curiously 

 similar are such parallels. I have already mentioned the very 

 evident resemblance between the water-boatman, the water- 

 beetles, and the human rower, the body of the insect being 

 shaped very much like the form of the modern boat. I must 

 now draw the attention of the reader to the similitude between 



