xiv.] FORBES* SCIENTIFIC WORK. 463 



iron plate of given thickness, whose faces are maintained 

 at constant given temperatures. As a proof of the 

 value attached by scientific men to these ingenious ex- 

 periments, it is only necessary to mention that the 

 British Association has given a grant for their repetition 

 with the best attainable instrumental means, and for 

 their extension to other substances than that to which 

 Forbes was obliged by failing health to confine himself. 



There is one point of Forbes' observations in this 

 matter which is so exceedingly curious and unlooked-for, 

 that in spite of Forbes* scrupulous accuracy and expe- 

 rimental skill, we must hesitate fully to accept it until 

 it is confirmed by other observers : especially as he may 

 have been led to it by the imperfection of his ther- 

 mometers, or the exceeding difficulty of properly con- 

 ducting this part of the experiment at very high 

 temperatures. The result in question is that, while (as 

 has long been known) the rate of cooling of a body by 

 simultaneous radiation and convection increases at first 

 ;id faster as its excess of temperature over that 

 of the air increases, this is true only for a range of 

 150 C., or so, after which the rate of increase becomes 

 slower and slower as the excess increases. In other 

 words, if a curve be drawn whose abscissae are the 

 a of temperature, and its ordinatos the corre- 

 sponding rates of cooling, this curve will be concave 

 upwards till about 1;3U ('., and will there have a point 

 of contrary flexure. 



Anoth .- -mely valuable investigation of For 



ft Tin- principle on 



which to proceed had, how< v< i\ been here dearly pointed 



out by Fourier; and had lieeii made use of by various 



6 and Belgium, and even by Sir John 



Leslie in this country. The value of Forbes' results is, 



rthelrss, very great. The five-year 

 vati the sets of thermometers buried in three 



lly <!iti' iterials near Kdinlmr^h ;. 



by C00Q iv important, information : and, M pa- 



!ed him to B l-dieve f,, r the first time, 



