METEOROLOGY OF BEN NEVIS IN CLEAR AND IN FOGGY WEATHER. 797 



1 a.m. and 12.30 p.m. respectively, and two minima at 7 a.m. and 6.30 p.m. 

 respectively. The shape of the curve produces the impression that immediately 

 the sun had set another sun rose. It will be observed that the diurnal heating 

 and cooling occupies twelve hours and the nocturnal heating and cooling occupies 

 the same length of time. The area contained between the curve and the base 

 line, forming a tangent to the minimum part of the curve, is proportional to the 

 heat added during the time corresponding to the interval between the initial and 

 final ordinates, and it is given directly in a convenient unit, namely, in Hour-degrees 

 (F.), which may be indicated by H.° F. Effecting the necessary measurements we find the 

 diurnal heating represented by 16*75 H.°F. and the nocturnal heating by 8*70 H.°F. 

 Thus the nocturnal heating is rather more than half of that produced by the sun 

 during the day. It is, therefore, roughly equivalent to what would be supplied by the 

 sun itself if it rose again after setting and shone for the same number of hours as 

 during the clay, eight hours ; attaining, however, only one-half of its meridian altitude 

 during the day. 



This nocturnal effect persists in the month of December, and, apparently, more 

 intensely relatively to the normal solar heating than in November. At the first glance 

 at the curve it would not be thought so, for there is no prominent rise and fall of the 

 curve during the night, as in November. The fall is then just before sunrise, but the 

 minimum and with it the fall and rise between the diurnal and the nocturnal heating is 

 almost obliterated, so that the curve, after rising in the forenoon to the normal 

 maximum in the early afternoon, hardly falls at all, and merges into the nocturnal 

 heating, forming a very flat curve, nearly parallel to the base line. 



We have seen from measurements on the November curve where tlie two effects are 

 sharply separated that the nocturnal heat supply is rather more than one-half of the 

 diurnal supply. If we calculate from Halley's # well-known formulae, the relative 

 amounts of heat communicated by the sun in a day in the middle of November and in 

 one at the middle of December in the latitude of Ben Nevis, we find that they are in 

 the proportion 356 : 189, or almost exactly in the proportion of the diurnal to the 

 nocturnal heating in November. If the nocturnal heating is the same in December as 

 in November, and as the night is longer it has every chance of being greater, the heat 

 due to the two sources would be equal, and the rise of temperature due to the one 

 would counteract the fall of temperature due to the other, producing a day with an 

 excessively small range of temperature. From the table the range is 1'4° F. The 

 mean temperature in December is 28*45°, or 4*29° below that of November. This 

 is a very small difference, especially when we find, from Lambert's t funda- 

 mental calculations of the yearly march of heat under different latitudes, that the 



* On the proportional Heat of the Sun in all Latitudes, with the method of collecting the same. By E. Halley, 

 Phil. Trans., 1693, vol. xvii. p. 878. Abridgment iii., p. 576. 



t Johann Heinrich Lambert's Pyrometrie oder vom Maasse des Feuers und der Wclrme, Berlin, 1779, p. 339 and fig. 35. 



