404 RECORD OF SCIENCE P'OR 1887 AND 1888. 



on the distance of the earth from the sun, but principally on the appar- 

 ent solar altitude above the horizon and on the length of the day. The 

 relative number of units of heat received by a horizontal surface, at the 

 outer surface of the atmosphere, in the course of twe itj-four liours is 

 shown for each month and each latitude either graphically or in the 

 numerical table No. i, which is here omitted as not necessary to this 

 abstract of the course of the argument. 



Tlie relative number of utnts of heat received at the outer surface of 

 the atmosphere during an entire year is shown by n. similar Table No. 

 II. According to Pouillet the total amount of solar heat received an- 

 nually at the outer surface of the atmosphere is equivalent to 231,075 

 calories per square centimeter, or an amount of heat that would melt a 

 layer of ice 30.89 meters thick, or evaporate into vapor a layer of water 

 about 4 meters deep, having a temperature of 5° C. (45° F.), or 3.9 

 meters, having a temperature of 27° C. (80° F.). The most recent 

 measures make the solar radiation about 20 per cent, larger than this. 



The relative amounts of solar heat received at the outer surface of 

 the atmosphere for the intervals between the three moments for whicli 

 the Signal Service tri-daily weather maps are made up, are given in the 

 numerical table, iii, and are also graphically shown on charts by lines 

 of equal amounts of solar heat received at all parts of the earth. 



(II) The results of insolation. — The effects of solar heat before and 

 after it reaches the earth's surface are analyzed as follows: 



A portion is (1) directly and selectively absorbed in its passage 

 through the earth's atmosphere to the surface of the earth, or to the 

 upper surface of a layer of fog or clouds, and does molecular work in 

 the air and vapor; the remaining portion passes directly, or after difiuse 

 reflection, finally to the earth's surface, and either (2) evaporates moist- 

 ure from the surfaces of the ocean, earth, clouds, leaves of plants, etc., 

 or (3) it heats up these latter and all other bodies, which, in turn com- 

 mniiicate the greater part of their heat by convection (4) and radiation 

 (5) to the adjacent air, although (6) a little is conducted down to a depth 

 of 50 or 100 feet, whence it returns subsequently. We will consider 

 these in detail. 



(I) Atmospheric absorption. — When the sun is near the zenith, about 

 25 per cent, (according to Pouillet) of the energy in its vertical raj's is 

 absorbed by the atmosphere before it reaches the earth's surfiice — the 

 quantity absorbed is loss in proportion as tlie air is purer and lighter, 

 and on the clearest days, at the summit of mountains 15,000 feet high, 

 the zenithal absorption is only about 20 percent. ; the difference is the 

 quantity absorbed in the lowest portion of our atmosphere, and which 

 is largely utilized in preventing the condensation of invisible aqueous 

 vapor. The researches of Langley indicate that much more than 25 per 

 cent, is absorbed by the atmosphere, that percentage applying only to 

 the rays that more easily ])enetrate the air, and that jiossibly as much 

 as 50 per cent, is the total absorption, as was indicated by Forbes in 1842. 



