1883.] 



S. A. Hill — The Measurement of Solar Radiation. 



taken only for the clear days in each month, but the means for all the days 

 here given, are practically the same in the case of barometric pressure, and 

 there is no difference in the pressure of vapour, of any importance, except 

 in the month of June. 



Table II. — Mean Barometric Pressure ; 2Q inches. 



Year. 



Jan. 



Feb. 



Mar. 



Apl. 



May 



June 



Oct. 



Nov. 



Dec. 



Mean for 9 

 dry months. 



1876 

 77 

 78 

 79 

 80 

 81 

 82 



•681 

 •795 

 •771 

 •723 

 •679 

 •779 

 •751 



•626 

 •736 

 •704 

 •646 

 •652 

 •690 

 •652 



•541 



•585 

 •599 

 •554 

 •509 

 •598 

 •557 



•389 

 •506 

 •478 

 •402 

 •390 

 •417 

 •416 



•300 

 •369 

 •380 

 •274 

 •305 

 •326 

 337 



•199 

 •236 

 •198 

 •210 

 •164 

 •219 

 •182 



•605 

 •601 

 •482 

 •546 

 •562 

 •519 

 •492 



•670 

 •678 

 •615 

 •686 



•727 

 •653 

 •689 



•787 

 •733 

 •693 

 •717 

 •768 

 •743 

 •711 



•533 

 •582 

 •547 

 •529 

 •528 

 •549 

 •532 



Mean. 



•740 



•672 



•563 



•428 



•327 



•201 



•544 



•674 



•736 



•543 



Table III. — Mean Tension of Vapour in inches of mercury. 



Year. 



Jan. 



Feb. 



Mar. 



Apl. 



May 



June 



Oct. 



Nov. 



Dec 



Mean for 9 

 dry months. 



1877 

 77 

 78 

 79 

 80 

 81 

 82 



•312 

 •428 

 •364 

 •325 

 •335 

 •284 

 •335 



•295 

 •349 

 •431 

 •354 

 •349 

 •357 

 •289 



•356 

 •450 

 •392 

 •356 

 •417 

 •431 

 •300 



•330 

 •534 

 •486 

 •325 

 •400 

 •433 

 •348 



•524 

 •592 

 ■630 

 •491 

 •650 

 •642 

 •469 



•744 

 •784 

 •640 

 •746 

 •823 

 •854 

 •855 



•678 

 •642 

 ■648 

 •704 

 •605 

 ■612 

 •635 



•447 

 •474 

 •454 

 •365 

 •409 

 •352 

 •428 



•343 

 •424 

 •318 

 ■306 

 •340 

 •306 

 •348 



•448 

 •520 



•485 

 •441 

 •481 



•475 

 •445 



Mean. 



'340 



•346 



•386 



•408 



•571 



■778 



•646 



•418 



•341 



•471 



Unless there be more than is generally admitted in Dr. Balfour 

 Stewart's theory that the occurrence of sunspots is determined or controlled 

 by the positions of the planets, it may be assumed that solar activity is not 

 subject to any important variation of a period equal to one of our years. 

 The monthly mean results of table I may therefore be taken to represent 

 the radiation of a mean sun modified only by terrestrial agencies ; and we 

 may proceed to apply the above modification of Pouillet's formula to these 

 monthly means, in order to find out the relative absorbing or scattering 

 effects of dry air, water vapour and dust or haze. The nine months give 



