MECHANIC POWERS. 35 



to one-tenth of a grain, it will require a consider- 

 able time to ascertain the weight to that degree of 

 accuracy, because the turn must be observed 

 several times over, and is very small. But if no 

 greater accuracy were required, and scales were 

 used which would turn with one-hundredth of a 

 grain, a tenth of a grain more or less would make 

 so great a difference in the turn, that it would be 

 seen immediately. 



10. If a balance be found to turn with a certain 

 addition, and is not moved by any smaller weight, 

 a greater sensibility may be given to the balance 

 by producing a tremulous motion in its parts. 

 Thus, if the edge of a blunt saw, a file, or other 

 similar instrument, be drawn along any part of the 

 case or support of the balance, it will produce a 

 jarring which will diminish the friction in the mov- 

 ing parts so much, that the turn will be evident 

 with one-third, or one-fourth of the addition that 

 would else have been required. In this way, a 

 beam, which would barely turn by the addition of 

 the tenth of a grain, will turn with' the thirtieth or 

 fortieth of a grain. 



• To those who are engaged in making nice philo- 

 sophical experiments, an accurate balance is of the 

 greatest importance. One of the best ever made 

 is that belonging to the Royal Society, executed 

 by the late Mr. Ramsden. 



The stater a, or Roman steel-yard, is a lever of 

 the first kind ; and is used for finding the weights 

 of different bodies, by one single weight placed at 

 different distances from the prop or centre of 

 motion D (fig. 14.) The shorter arm D G is of 

 such a weight as exactly to counterpoise the longer 

 arm D X. If this arm be divided into as many 



d °Z 



