PRESSURE OF AQUEOUS VAPOUR. IxxXvil 
3d, The intervals between the epochs of mean temperature and of the maximum temperature are nearly equal 
in spring and autumn; the afternoon interval is greatest in summer, and it is least in winter. If we except 
summer, the temperature increases as rapidly from the mean to the maximum, as it diminishes from the 
maximum to the mean ; the slight difference in autumn between the values of the intervals, and even that in 
summer may be due to error in the epoch of maximum resulting from the fewness of the observations. In 
each quarter, with the exception of summer, the temperature diminishes more rapidly after the maximum till 
sunset than it increased during equal time before the maximum.* It will be seen from the column for the year, 
in Table 75, that the mean temperature for the pairs of hours before and after 15 and 2" p.m., are equal or 
nearly equal till the pair 9" 10™ a.m. and 6 10™ p.m., which are also nearly equal, so that the mean diurnal 
eurve for the year from 94 10™ a.m. till 65 10™ p.m. is symmetrical about a vertical axis. 
PRESSURE OF AQUEOUS VAPOUR. 
TABLE 77.—Mean Pressure of Aqueous Vapour for each Month in the Years 1843-6. 
183. Annual Variation of the Pressure of Aqueous Vapour.—The pressure of aqueous vapour, as deduced 
from the observations of the dry and wet bulb thermometers, is least in February, being in the mean of 4 years 
= 0-198 inch of mercury, and itis greatest in August = 0°393 inch, the difference between the greatest and least 
monthly means being nearly two-tenths of an inch. The mean pressure for each of the four months December 
to March varies little ; so also for the four months June to September. 
The mean pressure for the four months December to March from 4 years’ observations = 0-211 inch. 
Deter ete nis nig « wks 0 vin qasie/e cag RBs aes o June to September... .-........-cecescereeeeee--oee == 0°381 
The mean pressure of aqueous vapour for the 4 years 1843-6 = 0:285 
TABLE 78.—Variations of the Pressure of Aqueous Vapour with reference to the Moon’s Age and 
Declination for the Years 1843-6. 
After 
Moon’s Moon 
Age. farthest 
North. 
a a. in. in, in. in. a a. 
14—16 5 : : : 27— 1 
17—20 : . 4 dj pee 
a 
21—24 || +. : . . 6— 8 
25—28 . F ; 9—12 
99 i || -. ‘ 5 : 13—15 
J Bila : 4 ; 16—19 
6— 9 ‘ -002 ‘ 20—22 
10—13 . : 23—26 
184. Variations of the Pressure of Aqueous Vapour with Reference to the Moon’s Age.—Though it has not 
been possible to determine by our apparatus the heating effect of the moon, yet it is believed that it has some 
f * The difference betwixt this result and that obtained by others is due, it is conceived, to the want of proper precautions to 
avoid the effects of radiation or reflection of the sun’s heat from the soil or surrounding objects in the afternoon. It will be seen, 
in the Introductions to the several volumes, that this source of error was cared for at Makerstoun. 
MAG. AND MET, oBs, 1845 anp 1846, y 
