141 



the other vessel being absorbed, displaced, or in a great degree driven 

 away by the colder covering of linen which closely embraced it. 



Led on by this conjecture, several experiments were made with 

 cylinders covered with one, two, or more coatings of glue, and of 

 copal varnish ; and the results, in fact, turned out favourable to the 

 supposition, the cylinder with one coat of glue losing 10 of its heat 

 in 43', and that with two coatings in about 38'. With the copal 

 also the cooling of the instrument became more and more rapid as 

 the thickness of the varnish was increased ; till, however, eight suc- 

 cessive coatings having been applied, the cooling again became less 

 rapid, and it was found that there was a maximum of thickness which 

 produced the greatest effect. No probable reason is yet assigned for 

 this limitation. 



The next object was to find out what effect colour would produce 

 in the experiments, and accordingly the cylinder with eight coatings 

 of varnish was painted black ; it was also painted in the same manner 

 after all the varnish had been washed away ; and lastly, it was like- 

 wise painted white : in each of these instances the cooling was ac- 

 celerated by the paint, nearly in the same proportion as in the pre- 

 ceding experiments. 



A nicety occurred now in the conducting of the experiments, which 

 was thought to deserve particular attention : though the apparatus 

 for confining che heat at the two flat ends of the cylinder was the 

 best that could be contrived, yet it is not at all unlikely that some 

 would escape in those directions, and thus occasion some fallacy in 

 the results. 



In order to investigate this point, a given number was previously 

 assumed as the measure of the whole quantity of heat emitted by 

 the whole instrument, without terminal coverings, during a certain 

 period. The surface of the whole of the cylinder was then accurately 

 measured, and also that of its vertical sides ; and thence was inferred 

 the proportion of heat that passed off through the sides of the instru- 

 ment, and what proportion must have escaped through its uncovered 

 ends. With these data it is easy to infer, from an experiment with 

 the ends covered, what proportion of the heat, lost in the cooling, had 

 escaped through the flat terminal surfaces when covered. In this 

 manner it has been ascertained that, assuming the total of the loss 

 of heat emitted by a cylinder, in a given time, for instance 55^', to 

 be 10,000, the quantity that escapes through the vertical sides will 

 be =7,015, and that which penetrates through the terminal sides 

 and coverings =2,985. 



Admitting these computations, it will now appear how an estimate 

 may be made, what proportion of the heat lost in any other experi- 

 ment, actually escaped through the vertical sides of the instrument : 

 and as the quantity of the heat emitted may well be represented by 

 the time of the emission, there can be no difficulty in substituting 

 the velocity for the quantity ; whence it is inferred, that in the ex- 

 periment, for instance, when the sides of one of the cylinders were 

 blackened, the velocity with which heat is given off from the naked 



