91 



the heat that it would receive if it were outside the atmosphere. 

 Of the remaining -25 per cent one-half reaches this horizontal unit 

 by way of the diffuse reflection from the sky, so that with the sun in 

 the zenitli the unit receives 0.875 of the original solar heat. For a 

 point on the equator during twelve hours this would amount to 

 O.STr>Xli^X60X()0 of the total possible if the sun were in the zenith. 

 Using this as a basal datum, Aymonnet obtains the relative numbers 

 given in the following table or the ratio of the heat actually received 

 during one day to that which would have been received if the sun had 

 stood for twelve hours in the zenith. Thus on June 20, at latitude H0°, 

 the horizontal unit receives 0.347 of that corresponding to the ideal 

 sun in the zenith all day, while at the north pole on the same day 

 the horizontal unit received during twenty-four hours 0.328 of what it 

 would had the sun stood in the zenith for twelve hours. In fact the 

 amount of heat received by horizontal surfaces is nearly uniform for 

 all latitudes for the days June 15-July 28. These relative numbers 

 or ratios may be turned into absolute calories by multiplying them 

 by the so-called " solar constant," whose value is probably between 

 two and three calories per minute per square centimeter. 



Relative q)iantities of total heat received on specified days from the sun and sky 

 at different latitudes hy a unit surface of horizontal (jround during one 

 cloudless day, allowing for the absorption and diffuse reflection of ordinary 

 clear air, as computed hy Aymonnet. 



Dates, 1874. 



Declin- 

 nation 

 of sun, 

 north. 



March 20 . 

 March 2K . 



April 7 



April 15. . 

 April 35.. 



May 5 



May 15.... 



May 25 20 



June 5 ;i2 



June 15 23 



June 19 to 23 23 



July 7 22 



July 19 20 



.Iuly28 



August 7 



August 18 



August 25 



September 5 . 

 September 15 

 September Zi 



Latitude— 



.279 

 .274 

 .273 

 .368 

 .267 

 .272 

 .274 

 .279 



0.295 

 .297 

 .303 

 .306 

 .307 

 .305 

 .:«4 

 .304 

 .303 

 .302 

 .301 

 .303 

 .304 

 .304 

 .;«5 

 .307 



.;«7 



.303 

 .297 



.310 

 .322 

 .:« 

 .337 

 .342 

 .346 

 .347 

 .342 

 .337 

 .331 

 .322 

 .310 

 .301 

 .284 



0. 175 

 .190 

 .215 

 .235 

 .291 

 .281 

 .298 

 .313 

 .324 



