and in the Condensation of Vapours. 377 



periment, and putting an end to the experiment as soon 

 as the water in the receiver has acquired a temperature 

 precisely the same number of degrees higher than the 

 air, the receiver will be heated by the air during half 

 the time of continuance of the experiment, and cooled 

 by it during the other half; so that the calorific and fri- 

 gorific effects of the air on the apparatus will counter- 

 balance each other, and produce no perceptible effect on 

 the results of the experiments; consequently they will 

 require no correction. 



When we are making experiments to elucidate natural 

 phenomena, it is always more satisfactory to avoid 

 errors, or to compensate them, than to trust to calcu- 

 lation for appreciating their effects. 



As the law of the variation of the specific heat of water 

 at different temperatures is not known, and as we have 

 but an imperfect knowledge of the true measure of the 

 intervals of temperature marked by the divisions of our 

 thermometers, to prevent the effects that our uncertainty 

 on these points would have on the subject of inquiry, I 

 took care to make my experiments in a room where the 

 temperature varied very little, and to confine them to a 

 few degrees of elevation of the temperature of the water 

 in the receiver. 



It is true, I made some experiments in a room where 

 the air was much colder, and in which I employed ice 

 instead of water to fill the receiver; but these experi- 

 ments were for a particular purpose, and are not classed 

 with the others. Besides, they never afforded such 

 uniform and satisfactory results as those made under 

 other circumstances. 



It has been fully proved, not only by the results of 

 my experiments, but by the experiments of others also, 



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