274 Pleasant Ways in Science. 



•of tliis movement read off on the edge of the circle by a 

 vernier. If all four legs are exactly of the same lengthy and 

 the instrument is placed on a plate of glass, or any other sub- 

 stance which is not a true plane, one or more of the legs will 

 not touch the surface when the others do, and if a slight 

 angular shove is then given to the instrument it will revolve 

 about the central leg if that leg touches any point, which . it 

 can easily be made to do. We took a plate of glass which 

 all four legs touched, and then we expanded a portion of the 

 glass by the heat of one or two fingers imposed upon it for a 

 minute. The particles of the glass experienced sufficient 

 motion to lift some legs of the instrument higher than the 

 others, and this extremely slight movement allowed us to 

 rotate the instrument about its central leg. This particular 

 instrument will measure inequalities not exceeding a fifty- 

 thousandth of an inch. 



Another kind of instrument, by which the amount of motion 

 performed in infinitesimal quantities of time can be estimated, 

 is called a chronograph. In this sort of apparatus there must 

 be a uniform rate of motion impressed upon one part, say a 

 wheel, and this part must transmit its motion through long 

 levers and through larger wheels, so that a very small motion 

 of the first part leads to a very much greater motion of some 

 other part. If one part moves through an infinitesimal space 

 for a hundredth, a thousandth, or a hundred thousandth of a 

 second, it must cause another part to move enough for the 

 eye, with or without optical aid, to be able to see and measure 

 the space that has been traversed. As an illustration of one 

 way of using such a machine, let us suppose that an electric 

 current suddenly sent through it sets it going, and at the same 

 moment fires off a gun. Let the ball strike and cut through 

 the wire conveying the current, and instantaneously stop the 

 machine. An observer can then see that the index hand of the 

 apparatus has moved through a space corresponding with a 

 given portion of time, say a thousandth of a second, and thus 

 he knows that from the firing of the gun to the ball's striking' 

 the wire a thousandth of a second elapsed. 



Motion is a necessary condition of life. In a living organism 

 everything moves. If new matter is taken in and digested we 

 have nothing more than a regulated series of motions, by means 

 of which food substances are taken to pieces in a methodic:") 1 

 manner, and their atoms built up in fresh forms. If old matter 

 is eliminated from the system, here again are motions regulated 

 as to their character, their quantity, and their velocity. If we 

 see, motion is communicated to the eye ; if we hear, motion is 

 communicated to the ear ; if we think or foci, motion affects the 

 brain. Countless myriads of regulated and co-ordinated motions 

 occur every instant that an animal lives. Change their order, 



