348 On the Construction and 



and with such instruments, a trifling change in the adjust- 

 ments may make ho great difference. The instruments to 

 which my remarks generally refer, and which I am in the 

 practice of using, are more than 60 times more sensible than 

 such as Dr. Ure alludes to. Even the beam which I am in 

 the habit of using in the analysis of minerals to weigh the 

 crucible with the results, for which despatch is necessary to 

 avoid waste of time, gives the 50-thousandth part, or shews 

 100 of a grain quite distinctly, moving with it though one 

 degree of a 7-inch arm when loaded with 500 grains. 



It has been generally allowed, that weights estimated by 

 equipoises at each end of a beam cannot be depended upon, 

 and therefore Borda introduced what is called " weighing by 

 substitution," for which purpose the substance to be weighed 

 is placed in one scale and exactly counterpoised by small 

 shot, &c. in the other. It is then removed, and weights are 

 placed in the scale until the beam is brought to the same 

 position as it had before, by which the weight is obtained in- 

 dependent of any error in the adjustments. 



To adjust a balance for this method of weighing, it is 

 unnecessary to make the zero point and the index agree in the 

 same horizontal line, nor is it necessary that the arms of the 

 beam should be exactly of the same length, nor the scale 

 pans exactly the same weight ; all that is required, is to make 

 the beam vibrate steadily and slowly, and to ascertain that 

 the points of suspension are nearly in the same line with the 

 centre of motion. 



The objection to this mode of weighing is, that it requires 

 a double operation, and as in using a fine balance 10 or 15 

 minutes are generally necessary, the time lost when several 

 weighings are required becomes of some consequence. To 

 remedy this I have used a modification of Borda 5 s method, 

 which may be called " supplementary weighing." In this 

 method a weight greater than is generally intended to be used 

 is placed in one scale pan and counterpoised by shot, &c. in 



