76 
Transactions of the Boyal Society of South Africa. 
TABLE 4. 
Meteorological Elements compared with the Corresponding 
Eanges of the 
Horizontal 
Pendulum. 
Cloudy 
Medium 
Clear 
Days. 
Days. 
Days. 
387 
634 
699 minutes 
47 
58 
61 degrees 
Maximum of earth temperature 
98 
106 
108 „ 
138 
139 
138 „ 
26-087 
26-098 
26-088 inches 
4-7 
5-9 
6-3 mm. 
266 
293 
288 days 
It appears from this that the range of the pendulum increases as the 
daily duration of sunshine increases. The same range is evidently inde- 
pendent of the variation of the diurnal range of pressure, and also of the 
direct heating power of the sun so far as the black bulb in vacuo is com- 
petent to measure it. That the range of earth temperature should depend 
upon the duration of sunshine is no more than would be expected, seeing 
that under a clear sky nocturnal cooling goes on very rapidly. It is not 
easy to see, however, why the maximum temperature just under the 
surface of the ground should be, on the whole, highest under clear skies. 
According to the indications of the black bulb, the greatest heating effect 
accompanies a sky about one-half clouded, and a very high temperature on 
the black bulb is as a rule accompanied by a high temperature of the 
surface soil. But of course the black bulb attains a maximum in a very 
few minutes, whereas the ground takes time to warm up, as shown by the 
fact that the mean maximum for the day at a depth of 1 inch occurs 
somewhere near 1} h. p.m. 
Table 5 gives the results of a comparison between the range of tem- 
perature of the surface soil, the duration of sunshine, and the mean daily 
range of the horizontal pendulum, arranged in sets according to the 
magnitude of the range of earth temperature. 
TABLE 5. 
Eange op Earth Temperature and Duration of Sunshine compared 
WITH THE corresponding EaNGE OF THE HORIZONTAL PeNDULUM. 
Cloudy Medium 
Days. Days. 
Eange of earth temperature 43 57 
Mean duration of sunshine 464 615 
Eange of pendulum 4-7 5-9 
* Photographic record. 
Clear 
Days. 
66 degrees 
657 minutes 
6-4 mm. 
