476 MR J. Y. BUCHANAN ON THE 



type is predominant in the winter months and the other in the summer months. For 

 this purpose the winter months are October to March, and the summer months April 

 to September. The principal feature which distinguishes the winter months from 

 the summer months in clear weather is the occurrence in the former of a pronounced 

 nocturnal heating effect. This shows itself particularly in the curves for November, 

 December, January, and February ; in October and March the passage between the 

 summer and winter types is apparent. 



Confining our attention more especially to the four months about midwinter, we 

 find the abnormal feature referred to very strongly marked in November. 



It 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. 



Nocturnal heating is observed in foggy weather in most of the months, and it takes 

 the form of a sharp rise of tempera.ture between midnight and 1 a.m., which then falls 

 gradually to a minimum at or near the hour of sunrise ; but there are exceptions to 

 this, as in November, and particularly in January, when the temperature rises very 

 uniformly from a minimum at midnight to a maximum at noon, and then falls again as 

 uniformly to the midnight minimum again. 



It must not be forgotten, in dealing with the winter months at Ben Nevis, that the 

 solar influence is very small. Lying in lat. 56°48' N. the sun's meridian altitude at the 

 winter solstice is only 9° 35', and the length of the day is under 6^ hours. It is, there- 

 fore, chiefly at this season that we might expect terrestrial or geographical influences to 

 produce their most apparent effect. With the march of the season the influence of the 

 sun increases very rapidly, and it has a tendency to obliterate the effects of other 

 agents, especially in the hours of the day when its heating power is increasing or 

 diminishing most rapidly. In the summer months, when the diurnal range of tempera- 

 ture is considerable, it is only in the neighbourhood of the epochs of maximum and of 

 minimum temperature that other influences can make themselves felt. At these times, 

 and especially at the time of maximum temperature, the heating and cooling influences 

 are for a time in a condition approaching equilibrium, during which the temperature 

 remains nearly constant, and its curve runs sensibly parallel to the line of time-absciss*. 

 Here we might expect other influences to show themselves ; and, in fact, if we inspect 

 the curves, especially those relating to clear weather, we see that most of them show 

 great irregularities in the neighbourhood of the extremes. Nearly all the clear weather 

 curves have strongly-marked irregularities near the date of minimum temperature, and 

 most of them, as February, March, August, and October, show similar irregularities near 



