xv.l ANALYSTS OF QUANTITATIVE PHENOMENA. 351 



of an error may be subject to continual variations, on 

 account of change of weather, or other fickle cirumstances 

 beyond our control. It may either be impracticable to 

 observe the variation of those circumstances in sufficient 

 detail, or, if observed, the calculation of the amount of 

 error may be subject to doubt. In these cases, and only 

 in these cases, it will be desirable to invent some artificial 

 mode of counterpoising the variable error against an equal 

 error subject to exactly the same variation. 



We cannot weigh an object with great accuracy unless 

 we make a correction for the weight of the air displaced 

 by the object, and add this to the apparent weight. In 

 very accurate investigations relating to standard weights, 

 it is usual to note the barometer and thermometer at the 

 time of making a weighing, and, from the measured bulks 

 of the objects compared, to calculate the weight of air 

 displaced ; the third method in fact is adopted. To make 

 these calculations in the frequent weighings requisite in 

 chemical analysis would be exceedingly laborious, hence 

 the correction is usually neglected. But when the chemist 

 wishes to weigh gas contained in a large glass globe for 

 the purpose of determining its specific gravity, the correc- 

 tion becomes of much importance. Hence chemists avoid 

 at once the error, and the labour of correcting it, by 

 attaching to the opposite scale of the balance a dummy 

 sealed glass globe of equal capacity to that containing the 

 gas to be weighed, noting only the difference of weight 

 when the operating globe is full and empty. The correc- 

 tion, being the same for both globes, may be entirely 

 neglected. 1 



A device of nearly the same kind is employed in the 

 construction of galvanometers which measure the force of 

 an electric current by the deflection of a suspended 

 magnetic needle. The resistance of the needle is partly 

 due to the directive influence of the earth's magnetism, 

 and partly to the torsion of the thread. But the former 

 force may often be inconveniently great as well as 

 troublesome to determine for different inclinations. Hence 

 it is customary to connect together two equally magnetised 

 needles, with their poles pointing in opposite directions, 



1 Regnault's Cours Elemental de Chimie, 1851, vol i. p. 141. 



