OF THE PROPERTIES OF MATTER. 19 



four brass studs B, C, D, L ; and two pins, one at F 

 and the other at H. On the pin F turns the crooked 

 index E /, and upon the pin H the straight index G K, 

 against which a piece of watch-spring R bears gently, 

 and so presses it towards the beginning of the scale 

 M N, over which the point of that index moves. This 

 scale is divided into inches and tenth parts of an inch : 

 the first inch is marked 1000, the second 2000, and so 

 on. A bar of metal O is laid into notches in the top of 

 the studs C and D; one end of the bar bearing against 

 the adjusting screw P, and the other end against the 

 crooked index El, at a 20th part of its length from its 

 center of motion F. Now it is plain, that however much 

 the bar O lengthens, it will move that part of the index 

 E I, against which it bears, just as far : but the crook- 

 ed end of the same index, near H, being 20 times as 

 far from the center of motion P as the point is against 

 which the bar bears, it will move 20 times as far as the 

 bar lengthens. And as this crooked end bears against 

 the index G 1C at only a 20th part of the whole length 

 G S from its center of motion H, the point S will move 

 through 20 times the space that the point of bearing 

 near H does. Hence, as 20 multiplied by 20 produces 

 400, it is evident that if the bar lengthens but a 400th 

 part of an inch, the point S will move a whole inch on 

 the scale ; and as every inch is divided into 10 equal 

 parts, if the bar lengthen but the 10th part of the 400th 

 part of an inch, which is only the 4000th part of an 

 inch, the point S will move the tenth part of an inch, 

 which is very perceptible. 



To find how much a bar lengthens by heat, first lay 

 it cold into the notches of the studs, and turn the ad- 

 justing screw P until the spring R brings the point 

 S of the index G K to the beginning of the divisions 

 of the scale at M : then, without altering the screw 

 any farther, take off the bar and rub it with a dry 



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