402 Mr. Poivell on Solar Light and Heat. [June, 



The surfaces of both balls being alike vitreous, they would be 

 equally affected by any simple heat. If, therefore, a glass screen 

 were so placed as to intercept the heat coming to one, but not 

 that impinging on the other, the difference exhibited between this 

 and the ordinary state of the instrument would make a very 

 minute quantity of simple heat conspicuous. If any heat were 

 thus intercepted on the interposition of the glass, the liquor 

 would obviously rise towards the plain bulb. This it might do 

 from the cooling effect of the glass ; but if we observe first wit li- 

 the glass (it having remained sometime with the instrument), the 

 effect would be perceived without this ambiguity on removing it. 



Another photometer of the " portable " kind had the upper 

 bulb coated with indian ink, and the lower washed with chalk. 

 With this similar experiments were repeated. I give the follow- 

 ing out of many which were all similar in their results. 



(19.) Large photometer. Bulbs, black and plain glass. 

 GraduationyVom the black bulb. 



Glass over 

 Both exposed. plain bulb two inches 



distance. 



9-30 a.m. 69° 70° 



60 60 



68 68 



2 p. m. 87 87-5 



88 88 



Advantage was taken of moments when the indication of the 

 instrument appeared tolerably stationary, which but rarely hap- 

 pens when it is used without its glass case. It here appears 

 that this instrument could not detect any sensible degree of heat 

 intercepted. 



(20.) Small photometer. Upper bulb, indian ink. Lower 

 bulb, washed over with chalk. Graduationy';o/rt the upper bulb. 



Glass over lower bulb, 

 half inch distance. Both bulbs exposed. 



(1.) 12° 15° 



17° 14° 18° 10° 



11° 14° 11° 13° 



20° 23° 20° 



(2.) 20° 22° 23° 24° 



21° 22° 21° 22° 23° 



23° 18° 18° 19° 



Here the fluctuations were more considerable than before ; 

 but on comparing all the results, it is obvious that the tendency 

 is to an increase rather than a decrease when the glass was 

 removed. This was probably owing to the glass acting in some 

 measure as interceptive of the heat radiating from the whitened 



