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Letter Ji'om Professor Leslie to the Editor on Mr Rltchie^s 

 Experiments on Heaty and New Photometer. 



My Dear Sir, 



JlLaving long projected the publication of a complete Trea- 

 tise on the Theory and Application of Heat, I have generally 

 overlooked such statements as have gone forth tending to limit, 

 modify, or contradict the principles I had already established, 

 being convinced that the precise and decisive experiments which 

 I shall produce, must dispel every shadow of doubt. My 

 anxiety to advance nothing except what was ascertained by the 

 most scrupulous accuracy, has hitherto retarded the appearance 

 of that work ; but I purpose, without further delay, to perform 

 the task thus imposed. 



In the mean time, I may stop to notice a circumstance which 

 has been sedulously turned against the doctrines which I had 

 propounded. If a red-hot ball be held behind a glass-screen, in 

 front of a metallic reflector, a considerable impression of heat is 

 concentrated at the focus ; from .which it has been hastily con- 

 cluded, that the calorific rays emitted from the ball (I borrow 

 the usual language, though it involves an assumption) pass 

 freely through the glass. But the fact is readily explained, 

 from the established principle, that the screen becoming much 

 heated, soon acts upon the reflector by its own radiation. Mr 

 Ritchie, Rector of the Academy at Tain, has, in a paper printed 

 in the first part of the Philosophical Transactions for the present 

 year, endeavoured to oppose this explication by some other ex- 

 periments. Suspending the hot-ball behind a very thin disc of 

 glass, he fotind a delicate thermometer placed before it to be 

 sensibly affected, though he kept blowing against the disc with 

 a bellows. Now, here lies the fallacy of the experiment ; for 

 the current would certainly not make the screen colder than the 

 air of the room, as Mr Ritchie supposes, but only prevent it 

 from acquiring so high a temperature as in a still atmosphere. 

 I have elsewhere shewn, that a wind of eight miles an hour only 

 doubles the dissipation of heat from the surface of a body. The 

 continual accumulation from the ball would therefore still enable 

 the disc to radiate profusely. 



