258 Instrument for determining 



pended by fine threads or wires from the end of the spring 

 atB. 



The substance whose specific gravity is to be ascertained 

 being placed in C, it will stretch the spring and lower the 

 index, suppose to E, where a mark is to be made; let the body 

 then be transferred to D, and a second mark F. made opposite 

 to where the index now stands. G being the point shewn by 

 the index when the scales are both empty, G. E. (= a) will 

 r^resent the whole weight of the above substance in air; G. F. 

 its weight in water, and E. F. (= (3) the difference of these 



weights; consequently ■>, := S, the specific gravity required. 



To find the value of S, take the extent E. F. with a pair of 

 compasses, and by means of them see how oft this is contained 

 in G. E. ascertaining the value of any fractional part by means 

 of a sector, or a scale of diverging lines as represented at H. 

 On a scale of this kind a horizontal line, as k, will be found, 

 equal to the divisor E. F, divided decimally, and with sub- 

 divisions, if of sufficient magnitude; here the amount of the 

 above fraction will be .71, and E. F. being twice contained in 

 G. E. 2.71 will be the value of S, or the specific gravity of the 

 substance experimented upon. 



Extreme accuracy is perhaps not to be attained by an instru- 

 ment of this kind without making the spring of an incon- 

 veniently large size; but for all common and generally useful 

 purposes it may be employed with advantage. If the spring is 

 of such dimensions that half a pound will without injuring it 

 stretch five or six inches, an error of one in five hundred can 

 scarcely occur, if common care is taken in ascertaining the 

 result. 



In the usual way of taking specific gravities, the imperfections 

 of balances sometimes employed, the chances of error in 

 noting down the weights, and in the consequent calculations, 

 however simple, frequently render repetitions necessary, and 

 employ a considerable portion of time. With an apparatus as 

 described above, time will be saved; and as the results are 

 made obvious in a great measure to the eye, as well as to the 



