1886.] Mr R. T. Omond on Temperatures at Ben Nevis. 25 
usual manner, is about the ordinary size; it measures 15 by 10 
by 15 inches. Readings of both sets of thermometers (high and 
low) were taken hourly during part of July and the whole of 
August. During the month of August, as well as these shade 
temperatures, readings were taken of a black bulb in vacuo , 
belonging to Mr H. N. Dickson, and kindly lent by him for the 
purpose. In this instrument the thermometer inside the glass 
globe, instead of being a maximum, is a common thermometer, 
with the bulb blackened in the usual way. The hourly readings 
of this instrument indicate the solar radiation at the time of 
observation, instead of, as in the usual maximum black bulb, giving- 
only the greatest intensity since being last set. In the first column 
of the table, at the end, the mean hourly values of this black bulb 
for the month of August are given. It is noteworthy that the 
maximum is almost exactly at twelve noon ; but, as Greenwich 
time is used, it must be borne in mind that the mean time of the 
sun’s meridian passage is about 12 hours 20 minutes, the longitude 
being some 5° west. In the second and third columns respectively 
of the table are the high and low level shade temperatures, and in 
the fourth the difference between them, for the month of August 
also. The values of the fourth column are shown graphically in 
the highest line of the diagram. The average temperature for the 
whole day is the same in both cases, but the upper thermometer 
has a slightly less daily range, reading about one-tenth of a degree 
F. higher at night, and from one to two tenths lower in the after- 
noon. The result is exactly what would be expected, the upper 
thermometer being further removed from the ground and less 
exposed to its radiation, while the smallness of the differences 
between the two can be accounted for by the exceedingly bad 
weather of last August : during the whole month there were only 
two fine days. A comparison of the readings on these two days 
(the 19th and 22nd), shows a much greater difference. In the fifth 
column of the table, the mean differences of the high and low level 
thermometers on the two days is given, and these numbers are also 
shown in the lower part of the diagram by the line which has the 
greatest range. Two days is of course far too short a period to 
give a satisfactory average and a smooth curve, but the general 
aspect of the line shows that, from about sunrise till noon, the 
