PLEASURES OF SCIENCE. 2J 



belongs Natural History, in so far as it describes the habits of animals 

 and plants, and its application to that department of Agriculture which 

 treats of cattle and their management. 



IV. But, for the purpose of further illustrating the advantages of 

 Philosophy, its tendency to enlarge the mind, as well as to interest 

 it agreeably, and afford pure and solid gratification, a few instances 

 may be given of the singular truths brought to light by the application 

 of mathematical, mechanical, and chemical knowledge to the habits of 

 animals and plants ; and some examples may be added of the more 

 ordinary and easy, but scarcely less interesting observations, made upon 

 those habits, without the aid of the profounder sciences. 



We may remember the curve line which mathematicians call a cycloid. 

 It is the path which any point of a circle, moving along a plane, and 

 round its centre, traces in the air; so that the nail on the felly of a 

 cart-wheel moves in a cycloid, as the cart goes along, and as the wheel 

 itself both turns round its axle, and is carried along the ground. Now 

 this curve has certain properties of a peculiar and very singular kind 

 with respect to motion. One is, that if any body whatever moves in a 

 cycloid by its own weight or swing, together with some other force 

 acting upon it, it will go through all distances of the same curve in 

 exactly the same time ; and, accordingly, pendulums are contrived to 

 swing in such a manner, that they shall describe cycloids, or curves 

 very near cycloids, and thus move in equal times, whether they go 

 through a long or a short part of the same curve. Again, if a body is 

 to descend from any one point to any other, not in the perpendicular, 

 by means of some force acting on it together with its weight, the line 

 in which it will go the quickest of all will be the cycloid, not the straight 

 line, though that is the shortest of all lines which can be drawn between 

 the two points; nor any other curve whatever, though many are much 

 flatter, and therefore shorter than the cycloid but the cycloid, which is 

 longer than them, is yet of all curves or straight lines which can be 

 drawn, the one the body will move through in the shortest time. Sup- 

 pose the body is to move from one point to another, by its weight and 

 some other force acting together, but to go through a certain space, as 

 a hundred yards, the way it must take to do this in the shortest time 

 possible, is by moving in a cycloid ; or the length of a hundred yards 

 must be drawn into a cycloid, and then the body will descend through 

 the hundred yards in a shorter time than it could go the same dis- 

 tance in any other path whatever. Now, it is believed that birds which 

 build in the rocks, drop or fly down from height to height in this course. 

 It is impossible to make very accurate observations on their flight and 

 path ; but there is a general resemblance certainly between the course 

 they take and the cycloid, which has led ingenious men to adopt this 

 opinion. 



If we have a certain quantity of any substance, a pound of wood, for 

 example, and would fashion it in the shape to take the least room, we 

 must make a globe of it ; it will in this figure have the smallest surface. 

 But suppose we want to form the pound of wood, so that in moving 

 through the air or water it shall meet with the least possible resistance, 

 then we must lengthen it out for ever, till it becomes not only like a 



