322 Mr Ritchie on Leslie? s Photometer. 



such a distance from the scene of action, I cannot be suspect- 

 ed of partiality to either party, and shall, therefore, calmly 

 express my conviction of the truth, without the least appre- 

 hension of giving offence to Mr Leslie, whose love of philo- 

 sophic truth is so ardent, that I am convinced he will be the 

 first to acknowledge the justice of the following observations, 

 or to point out the errors into which I may have fallen. 



The photometer is merely a differential thermometer, hav- 

 ing one of its balls blown of black enamel, while the other is 

 blown as thin and transparent as possible. The whole is then 

 inclosed in a transparent glass case. The black ball intercepts 

 the greater portion of incident light, whilst the transparent 

 one allows the greatest part to pass freely through. The 

 light which is thus intercepted by the black ball, is gradually 

 conducted to the interior, and thus expands the contained air. 

 As the black ball is placed considerably above the transparent 

 one, (in the portable photometer,) the ambient air will con- 

 tinue to receive fresh accessions of heat, till the expendi- 

 ture from the exterior surface of the glass is equal to the in- 

 crement of light which the black ball intercepts. Now, as 

 this quantity is partly carried off by radiation, and by the 

 conducting and carrying powers of the air, it must vary with 

 the surrounding sky, and with the density of the atmosphere, 

 &c. A cold sweeping wind has also a powerful effect in car- 

 rying off the accumulated store of heat. These causes are so 

 variable, that though the sun were to shine constantly with 

 the same splendour, and remain in the same situation, the in- 

 dications of the photometer would be extremely various. To 

 be convinced of this, place the photometer opposite the sun, 

 in a calm sheltered situation, and then remove it quickly to an 

 elevated place, where it is exposed to a chilly wind from the 

 north, and the number of degrees will be found to be less than 

 formerly, even though the sun continued to shine with un- 

 clouded aspect. But, the instrument, when applied to measure 

 the intensity of the sun's rays, is subject to another inaccu- 

 racy, which must have, considerable influence in changing the 

 result. The reflected light from the clouds and from the 

 earth, mingles its effect with the direct radiation from the sun, 

 so that we can deduce no conclusion whatever from the indi- 



