iiO Mr. B. VoweW's expet-ii/u'/itul Inqui)y into the Nature 



in the inverse ratio of their diameters, would diminish the 

 ratio of their risings. That this diminution was very trifling, 

 and not at all sufficient to account for the observed difference 

 of ratio will be evident, becau:^e the 1st set was made without 

 employing the box, the thermometers being suspended at a 

 distance from any object which could radiate heat to them ; 

 and in this set the difference of ratio is quite as conspicuous. 

 This remark applies likewise to the possible communication 

 of heat by the air. 



(10.) We must also take into consideration the effect due 

 to the glass screen. When we consider the two bulbs as 

 heated only by that part of the radiation which is transmitted 

 through the screen, the screen may be regarded simply as a 

 third body placed near the two bulbs; and whether it pos- 

 sesses a higher or a lower temperature, there will be a ten- 

 dency to bring all three to an ecjuality in proportion to the 

 difference of temperature, and in the bulbs, dependent on their 

 diameters modified by the state of their surfaces. This effect 

 arises from simple radiant heat ; whilst that derived from the 

 luminous hot body, is evidently following a different law with 

 regard to the surfaces. It will easily follow, from what has 

 been already shown, that such a secondary heating eflect will 

 be of a kind tending to diminish the ratio otherwise obtaining; 

 between the effects on the two bulbs. If the effect were of a 

 cooling nature, the same thing would also take place ; for I 

 ascertained that the radiating jjowers of the coatings em- 

 ployed, deduced from the observed rates of cooling, were in a 

 ratio which happened to be almost exactly the inverse of that 

 of the diameters ; but this effect is probably always small ; and 

 I have roughly allowed for it, as will be seen immediately; 

 taking the temperature of the screen by a small thermometer 

 having its bulb in contact with the central part of the surface. 



II. (11.) I now proceed to state the results, which will be 

 most conveniently exhibited in a tabular form. 



1st Set. Incandescent iron. Distance 7 inches. 



No 



