Sir Benjamin Thompson’s 
52 
merits of which I am now about to give an account, I have in 
general reversed the operation ; that is to say, instead of 
observing the times of heating, I have first heated the body 
in boiling water, and then plunging it into a mixture of 
pounded ice and ice-cold water, I have noted the times taken 
up in cooling. 
I have preferred this last method to the former, not only on 
account of the greater ease and convenience with which a 
thermometer, plunged into a mixture of water, may be ob- 
served, than when placed in a vessel of boiling water, and 
surrounded by hot steam, but also on account of the greater 
accuracy of the experiment, the heat of boiling -water varying 
with the variations of the pressure of the atmosphere: con- 
sequently the experiments made upon different days will have 
different results, and of course, strictly speaking, cannot be 
compared together; but the temperature of pounded ice and 
water is ever the same, and of course the results of the experi- 
ments are uniform. 
In heating the thermometer, I did not in general bring it to 
the temperature of the boiling water, as this temperature, as 
I have just observed, is variable ; but when the mercury had 
attained the 75 0 of its scale, I immediately took it out of the 
boiling water, and plunged it into the ice and water ; or, which 
I take to be still more accurate, suffering the mercury to rise 
a degree or two above 75 0 , and then taking it out of the boil- 
ing water, I held it over the vessel containing the pounded ice 
and water, ready to plunge it into that mixture the moment 
the mercury, descending, passes the 75 0 . 
Having a watch at my ear which beat half seconds (which 
I counted), I noted the time of the passage of the mercury 
