466 MR J. Y. BUCHANAN ON THE 



time that it is above the horizon each day. The heating power of the sun on any 

 horizontal portion of the earth's surface varies with the sine of its altitude at the 

 moment. All the elements of climate and weather depend ultimately on this factor, 

 and its variability produces a corresponding variability in the weather. It is only 

 necessary to consult a table of sines to see where the greatest variability is likely to 

 occur. Thus, at the equator the sine of the sun's meridian altitude varies between 

 0*94 and 1*000, at either of the Tropics between 0*684 and 1*000, in lat. 45° between 

 0*367 and 0*930, and in the latitude of Ben Nevis (56° 48' N.) it varies between 

 0*169 and 0*835. At the equator the sun's heating power at noon only varies by 

 7^ per cent, of its maximum amount, while at Ben Nevis the variation is 80 per cent. 



But the latitude of a place determines not only the intensity of the sun's heat 

 which it receives, it also determines the intensity of the cooling to which it is exposed 

 by radiation into the upper regions of the atmosphere and into space. This goes on 

 all day independently of the sun's heating, but it becomes more apparent after the 

 sun has set, and produces the greater effect the longer is the duration of the night, 

 and this is a function of the latitude. While the altitude which the sun attains 

 measures the heat which it supplies, and the length of the night determines the amount 

 which is lost, both the heat received and that lost at any particular place may be in 

 excess or in default of what it is entitled to, owing to its latitude alone. This is due 

 to secondary actions set up by the primary heating and cooling, whereby one place may 

 receive, in addition to its own, a supply of heat or of cold to which it is not entitled, 

 thereby altering its climate as well as that of the other place which has supplied the 

 heat or cold. 



The principal secondary agencies through which the sun works are the atmosphere 

 in its motion, both horizontal and vertical, and in its changes of volume ; and the sub- 

 stance of water in its changes of state of aggregation. It cannot be doubted that, con- 

 sidering the Earth as a whole, the nett effect of these secondary agencies is nil. Yet 

 their local effect is in many cases very great, as witness the difference in population 

 between Great Britain and places in the same latitude in North America. The 

 waters of the ocean are also a powerful secondary agency in the distribution of the 

 sun's heat. 



The published observations were begun on Ben Nevis in 1884, but, to avoid a 

 broken series, the years from 1st January 1885 to 31st December 1897 are here 

 considered. 



The principle on which the dates were selected was : — For foggy weather, to take 

 spells of three or more whole days of continuous foggy weather, and continuous foggy 

 iveather is defined by twenty-four consecutive entries of fog in the log of each day. 

 The supply of foggy days seemed to be so abundant that the minimum length of spell 

 was able to be fixed at three whole days. When it became a question of selecting the 

 spells of clear weather it was necessary to adopt the hour as unit, and twenty-four 

 consecutive hours during which fog was not once entered in the log was adopted as the 



