29 



to 2 Ibs., the larger values being remote from the point of sup- 

 port ; that on the longer arm is a light one, and reads from 

 to 48 parts of an ounce, the larger values being nearest the 

 point of support. The substance to be weighed thus balances the 

 larger rider by the aid of the smaller one,, and if with the 

 latter at it more than balances the larger when this is placed 

 at the figure representing the next lower ounce to its weight, it 

 will exactly balance it, as the leverage of the small one is reduced 

 by moving it to a higher graduation towards the point of support. 

 The readings of the two riders then give the weight. 



87. Parish's specific-gravi by balance. 



Lent by the Geological Laboratory, Royal College of 

 1887. Science. 



This is very much on the same principle as Attwood's, but is 

 adapted for small specimens only. It works on a knife-edge, and 

 carries a pointer and graduated arc. The object to be weighed 

 is suspended in the middle of the shorter arm by a fixed hook, 

 which carries two scale pans, one below the other, for weighing 

 in water in a beaker. The long arm supports a scale pan 

 hanging from a rider, and is graduated. There is also a small 

 rider on the same side as the point of suspension of the substance 

 to be weighed. The weighing takes place in the same way as in 

 Attwood's, but owing to the difference of weights which may be 

 placed on the scale pan of the long arm, the range of weighing is 

 proportionally greater, or, in other words, the determination is 

 more accurate for small objects. The too great oscillation of the 

 beam is prevented by its working in a limiting slot, in a support 

 at the remote end. 



88. Cohen's balance for determining the specific- 

 gravity of heavy liquids, used for the isolation of 

 minerals in rocks. Made by Gr. "Westphal, Celle. 



Lent by the Geological Laboratory, Royal College of 



1891. Science. 



This is a small balance constructed of brass, with levelling 

 plate, and a draw tube, to allow of adjustment for height. The 

 weighing is effected, as in the steel-yard, by the shifting of a 

 weight along the graduated beam of the balance. The instrument 

 is supplied with glass tube for the liquid, and a float with ther- 

 mometer, attached to the fend of the beam for suspension in the 

 substance under examination. A set of riders allows of the 

 measurement of specific gravities of liquids lighter and heavier 

 than water. With these, results can be obtained perfectly 

 correct to three places of decimals. 



U 69149. C 



