Mechanical Science. 389 



the weight. The hook of the latter then rests at the specific 



gravity on the scale. 



The demonstration of this is very simple. The instrument being 



supposed in equilibrium, and the shorter arm, and the weight of 



the counterpoise being constant, the weight of the body varies as 



the distance of the counterpoise from the point of suspension or 



B by the common principle of the lever. Hence if C be the place 



of the weight at the conclusion of the operation 



Weight in water : weight in air : : BC : BA, A being the end of 



the lever, and, by subtraction, the loss of weight in water : weight 



\n ATI J I. weight in air AB .„ 



m air : : AC : AB : and hence 2 — = specific 



loss AC 



gravity ; which is the rule. q. e. d. 



Substances lighter than water may have, if necessary, their 

 specific gravity ascertained by ihe usual method ; a scale of 

 equal parts being cut on the opposite end of the beam, and the 

 article to be weighed placed in a notch for the purpose. For 

 mineralogy, however, this will seldom be necessary. The 

 bottom of the notch A (at the smaller end) should be in a line 

 with the edge of the scale, its sides being a little raised. 

 The top of the shorter end should be rather the thickest part of 

 it to allow the horse-hair, by which the mineral is suspended, 

 to swing clear. This mode will be found very delicate and 

 accurate, and a hook must not be used, as it cannot be 

 balanced. 



The instrument in this form is exceedingly compact, and 

 may be reduced to a rod. The principle is capable of being 

 applied to an arc of a circle, witii a rod, resembling in its 

 application a common bent lever. 



2. Trigonometrical Surveys. — For some years past, a tri- 

 gonometrical operation has been conducted in India, under the 

 auspices of the local governments. Lieut.-Col. Larnbton has 

 been enabled, by the aid of their proceedings, to measure, at 

 different periods, an arc of the meridian from 8° 9' 38" to 

 18'' 3' 23" of north latitude, the greatest that has been niea- 

 ■^ured on the surface of the globe. From a review of the ope- 



