406 THE PRINCIPLES OF SCIENCE. 



remains the method of correction which was beautifully 

 carried out by Eegnault in his determination of the latent 

 heat of steam. He employed two calorimeters, made in 

 exactly the same way and alternately used to condense a 

 certain amount of steam, so that while one was measuring 

 the latent heat, the other calorimeter was engaged in 

 determining the corrections to be applied, whether on 

 account of radiation and conduction from the vessel or 

 on account of heat reaching the vessel by means of the 

 connecting pipes d . 



4. Method of Compensation. 



There are many cases in which a cause of error cannot 

 conveniently be rendered null, and is yet beyond the 

 reach of the third method, that of calculating the requisite 

 correction from independent observations. The magnitude 

 of an error may be subject to continual variations, on 

 account of change of weather, or other fickle circumstances 

 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 any 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 



d Graham's ' Chemical Reports and Memoirs,' Cavendish Society, pp. 

 247, 268, &c. 



