DIURNAL VARIATIONS OF THE PRESSURE OF THE WIND. xevil 
TABLE 92.—Diurnal Variation of the Maximum Pressure of the Wind within 10™ at the Observation 
Hours, for each Astronomical Season and for the Year, deduced from the Observations of the Years 
1843-6. 
Mak. Nov., Feb., May, 
Mean Dec., March, | June, Mean Dec., March, June, Sept., Year. 
Time. Jan. April. July. Time Jan April July. Oct, 
eae 0: 1b. 1b. Ib. ie) an: 1b. 1b 1b. 1b lb. 
12 10 || —0-07 | —0-21 | —0-20 0 10 || +0-13 | +0-25 | +0-23 | +0-17 || +0-19 
13 10 | —0-10 | —0-15 | —0-19 1 10 | +0-11 | +0-29 | +0-27 | +0-22 || +0-22 
14 10 || —0-13 | —0-14 | —0-21 2 10 || +0-06 | +0-32 | +0-28 | +0-19 || +0.21 
15 10 || —0-03 | —0.08 | —0-22 3 10 || +0-02 | +0-26 | +0-25 | +0-14 || +0-17 
16 10 || —0-05 | —0-10 | —0-18 4 10 || —0-03 | +0-14 | +0-18 | +0-11 |} +0-10 
17 10 || —0-02 | —0-12 | —0-15 5 10 || +0-02 | +0-03 | +0-14 | +0-02 || +0-05 
18 10 || —0-05 | —0-08 | —0-11 6 10 || —0-03 | —0-05 | +0-03 | —0-05 || —0-02 
19 10 || —0-05 | —0-10 | —0-03 7 10 || —0-01 | —0-15 | —0-06 | —0-07 || —0-07 
20 10 | —0-03 | +0-03 | +0-13 8 10 || +0-02 | —0-19 | —0-13 | —0-10 || —0-10 
21 10 | +0-04 | +0-11 | +0-17 9 10 || —0-02 | —0-14 | —0-19 | —0-10 || —0-11 
22 10 | +0-07 | +0-21 | +019 10 10 || +0-02 | —0-16 | —0-21 | —0-13 || —0-12 
23 10 | +0-11 | +0-24 | +0-23 11 10 0-00 | —0-21 | —0-19 | —0-08 || —0-12 
202. Diurnal Variation of the Mean Pressure of the Wind.—It is evident from the means in Table 92, 
that 4 years’ observations are too few to destroy the irregularities produced by the large atmospheric disturb- 
ances ; the following, however, are the approximate epochs of maximum and minimum :— 
Minimum. Mean. Maximum. Mean. 
Winter, Noy., Dec., Jan., 2» 10™ a.m. 8 36™ a.m. 02 10™ p.m. 3" p.m.—11» p.m. 
Spring, Feb., March, April, 115 40™ p.m. 75 56™ a.m. 14 55™ p.m. 5b 32m p.m. 
Summer, May, June, July, 9) p.m.—4" a.m. 7> 21™ aM. 1h 45™ p.m, 6 30™ pM. 
Autumn, Aug., Sept., Oct., 8? P.m.—7> a.m. 85 327 a.m. 1» 0™ p.m. 5h 97m px, 
Year, 12 aM. 7h 57™ am. 1 30™ p.m. 5) 53m pm. 
It will be seen that the variation of the pressure of the wind obeys a law analogous to that of the variation of 
temperature, while the sun is above the horizon ; it follows the ascent and descent of the sun, however, more 
closely than the temperature: thus, in winter the mean pressure of the wind occurs almost exactly at sunrise 
and at sunset, and the maximum occurs immediately after mid-day, in all instances anticipating the correspond- 
ing epochs for the temperature by an hour or more. A similar difference is observable in each quarter; this 
will be seen most satisfactorily in Plate [X., where the dotted curves of wind pressure are projected on the same 
mean or zero lines; as the curves for the temperature of the air : while the day portions of the curves are evi- 
dently connected with each other, this is not the case during the night ; the minimum pressure of wind has upon 
the whole the same relation to midnight that the maximum has to mid-day. In winter the pressure varies 
irregularly from hour to hour during the night; in summer and autumn the pressure is nearly constant for 
some hours before and after midnight. When we consider the mean for the year, we find the ordinates of 
_ equal value in the day portion of the curve at times equidistant from 14 p.m., and in the night portion of the 
_ curve, from 15 a.m.: the pressure of the wind, therefore, is related more directly to the position of the sun than 
_ to the temperature of the place, especially during the night. 
a The range of the mean diurnal variation is least in winter, and it is greatest in spring and summer—the 
_ ranges are— 
Winter = 0-26 Ib. Spring 0:53 Ib. Summer 0:50 Ib. Autumn 0°36 Ib. Year 0-37 Ib. 
