34 



S. A. Hill — Ohservations of the Solar Thermometer. [No. I, 



nearly 50 j)er cent, in tlie deduced coefficient. Nevertheless, since it is 

 impossible in practice to apply the method of prismatic analysis to all 

 the very numerous actinometric observations which are required to prove 

 the constancy or otherwise of the sun's radiation, and since the results 

 of such an analysis must necessarily be vitiated to some extent by the 

 selective absoption of the prism, some simple formula must be nsed and 

 that of Ponillet is the best hitherto suggested. Even if the atmospheric 

 absorption and consequently the radiation before it enters the atmos- 

 phere, as determined by this formula, be both much less than they ought 

 to be, their variations from day to day or from month to month deduced by 

 means of the formula must be in the same direction as they are in reality. 

 Ponillet's formula is r = Ap^, where r is the observed heating effect, 

 A the effect undiminished by atmospheric absorption, p the diathermancy 

 or transmission coefficient, and e the thickness of the atmosphere tra- 

 versed by the rays. Table II. gives the values of e which have been 

 employed in reducing the Lucknow observations. They have been calcu- 

 lated to a first approximation only, that is to say, they are equal to the se- 

 cants of the sun's zenith distance at the hours of observation. 



Table II. — Atmospheric Thiclcness at Luchiow, Latitude 26° 50' N. 







Hours of observation, Mean Time, 



Date. 

























8 



9 



10 



11 



12 



13 



14 



15 



IS 



January 



., 7th 



4-44 



2-52 



1-89 



1-62 



1-53 



1-59 



1-81 



2-33 



3-86 





14th 



4-38 



2-49 



185 



1-58 



1-50 



1-54 



1-75 



2-23 



3-38 





21st 



4-29 



2-43 



1-81 



1-55 



1-46 



1-50 



1-69 



213 



3-23 





28th 



4-15 



2-34 



1-75 



1-49 



1-41 



1-44 



1-62 



201 



306 



February . . . 



.. 7th 



3-79 



2-21 



1-62 



1-43 



1-35 



1-38 



1-54 



1-89 



2-80 





14th 



3-55 



212 



1-61 



1-38 



1-30 



1-33 



1-47 



1-81 



2-61 





21st 



3-25 



2-00 



1-54 



1-33 



125 



1-28 



1-42 



1-74 



2-48 





28th 



2-99 



1-89 



1-47 



1-27 



1-20 



1-23 



1-36 



1-66 



2-34 



March 



.. 7th 



2-71 



1-78 



1-40 



1-23 



117 



1-20 



1-33 



1-61 



2-25 





14th 



2-51 



1-69 



1-34 



119 



113 



117 



1-29 



1-57 



217 





21st 



2-34 



1G3 



1-31 



116 



110 



115 



1-27 



1-55 



211 





28th 



219 



1-55 



1-26 



112 



1-08 



111 



1-23 



1-49 



204 



April 



.. 7th 



203 



1-47 



1-21 



109 



1-05 



1-09 



1-21 



1-45 



1-97 





14td 



1-93 



142 



118 



107 



1-04 



1-07 



119 



1-43 



1-93 





21st 



1-86 



1-39 



1'16 



1-06 



103 



1-06 



117 



1-41 



1-89 





28th 



1-80 



1-36 



114 



1-04 



101 



105 



116 



1-39 



1-86 



May 



.. 7th 



1-74 



1-33 



113 



103 



101 



104 



1-15 



138 



1-82 





14th 



1-71 



1-32 



112 



103 



1-00 



1-04 



115 



1-36 



1-79 





21st 



1-69 



1-31 



112 



103 



1-00 



1-04 



114 



1-35 



1-77 





28th 



1-69 



1-31 



112 



103 



1-00 



103 



113 



1-34 



1-74 



