432 Dr. Blagden's Supplementary Report 
that presented itself. On this occasion, Mr. Cavendish was 
so good as to mention some experiments made by his father. 
Lord Charles Cavendish, with instruments on that con- 
struction, for the very purpose of determining the expansion 
of fluids ; and other experiments, of the same nature, have 
appeared in print. The application of this method, how- 
ever, was thought liable to a most important objection, from 
the great difficulty there is of being sure that the spirits in the 
ball are exactly of the temperature indicated by the thermo- 
meter placed by the side of it. To enlarge upon this cir- 
cumstance would be useless, as every person accustomed to 
experiments is aware, that all possible precautions, joined to 
the utmost attention in the observer, are scarcely sufficient to 
ensure this essential correspondence of temperature ; which 
reason alone would have induced us to prefer the method 
by weight. But there was another argument which still 
more forcibly determined us in favour of the latter ; namely, 
that the effect of mixture was found in that way, and therefore 
we were sure it admitted of as great accuracy as was obtained 
in the other part of the experiments. Greater nicety, if there 
had been a method which allowed of it, would have been 
superfluous ; and to incur the risk of less accuracy would have 
been absolutely unjustifiable. By using the same method to 
determine all the changes of specific gravity, those from heat 
as well as those from mixture, an uniformity is given to the 
whole series of experiments, and no one part of the results is 
liable to more suspicion than another. 
Till this time, I believe, the instruments with a ball and 
tube, for trying expansions, had all been constructed in the 
manner of real thermometers, to be filled by means of heat j 
