106 Mr Ritchie's Observations on Leslie's Photometer. 



permeates the case, will vary with the different altitudes of 

 the sun, though the quantity of incident light should remain 

 uniformly the same. It is also obvious, that the light, reflected 

 from the interior surface of the case to the balls of the instru- 

 ment, will also vary with the altitude of the sun. If the pho- 

 tometer be carried to the torrid zone, the incident light being 

 then in the direction of the two balls, the black ball will act 

 as a screen to the transparent one, and thus cause the liquid 

 in the stem to descend farther than it would do, if the trans- 

 parent ball were also exposed to the direct rays of the sun. 

 Another inaccuracy results from the variable quantity of light 

 absorbed by the transparent ball, arising from the different 

 altitudes of the sun. When the rays fall upon the instrument, 

 in a direction nearly horizontal, they will find a ready passage 

 through the thin transparent hemispheres ; but when the sun 

 is considerably elvated, a large portion of the incident light 

 will be absorbed by the lower hemisphere of the transparent 

 ball, in consequence of its connection with the tube. The 

 transparent ball will therefore, in this position, absorb a great 

 proportional part of the whole incident light, and occasion an 

 error of considerable magnitude. The results of different 

 photometers cannot therefore be compared, as they have no re- 

 lation to an invariable standard. 



The instrument in this form cannot, therefore, be regarded 

 as a photometer, but merely as a photoscope. But though 

 philosophers can place no confidence in its results, it must ne- 

 vertheless be considered as a very ingenious and elegant in- 

 strument. Though it does not mark with numerical accuracy 

 the dilute shadings of light, " the photometer nevertheless ex- 

 hibits distinctly (as the Professor poetically expresses it) the pro- 

 gress of illumination, from the morning's dawn to the full vi- 

 gour of noon, and thence its gradual decline, till evening has 

 spread her sober mantle ; it marks the growth of light from the 

 winter solstice to the height of summer, and thence its gradu- 

 al decline through the dusky shades of autumn." 



One of the greatest desiderata in meteorology at present 

 seems to be, an accurate and expeditious method of compar- 

 ing the calorific effects of the sun's rays in different latitudes, 

 at different elevations, and at different periods. Mr Daniell, 



