ANNUAL VARIATIONS FOR THE ATMOSPHERIC PRESSURE. xi 
190. Annual Variution of the Atmospheric Pressure.—Eight years’ observations appear insufficient for an 
accurate determination of this law. If we give the monthly means for each year equal weight, we find the 
probable error of the means in the last column of Table 83 to be . 
Jan. “% Feb. March. April. May. June. July. Aug. Sept. Oct. Noy. Dec. 
in. in. in. in. in. in. in. in. in. in. in. in. 
0-043 0:045 0:087 0:040 0-028 0-020 0-016 0-021 0:024 0-030 0:025 0-044 
The probable errors of the means for the five months December to April are greatest, and they are least 
for the months June, July, and August.’ The irregularity of the monthly mean pressure is therefore least at 
the hottest season, and greatest at the coldest season of the year ; it does not vary, however, with the irregu- 
larity of the monthly mean temperature. (See No. 178.) It is evident from these probable errors that the 
accurate epochs cannot be obtained from the last column of Table 83. If we take the means of each couple 
of months, the probable error of each mean will be reduced to about a half (the probable error of the mean 
of December and January, = 0:024 inch, of January and February, = 0-019 inch, &c.), and the annual law 
will be moré certain ; these means are as follow :— 
Prefix Jan.—Feb.— March— A pril—May—J une—July— Aug.—Sept.—Oct.—Nov.—Dee.—Jan. 
29 in. 548 -568 -601 -666 -683 +643 -636 -685 -631 -527 :581 -600 
These numbers give nearly the same result as. that derivable from the simple means in the last column of 
Table 83. It is probable, therefore, that at Makerstoun the atmospheric pressure is a maximum from May to 
September, being rather less for the intermediate months than for the first and last of that period; that it is 
a minimum in the end of October and in the beginning of February, a secondary maximum occurring in the 
end of December.* 
191. The quarterly groups which give the greatest range of mean pressure are the following,— 
Jan..Feb. Mar, April, May, June, July, Aug. Sept. Oct. Nov. Dec. | Oct.—Mar. Apr.—Sept. 
in. in. in, in. in. in. 
29561 29-660 29-669 29°569 | 29-565 29°665 
~ 
* Having examined the excellent series of barometric observations made under the direction of the Astronomer Royal at 
Greenwich, simultaneously with the Makerstoun series, for the purpose of comparing the annual law at the two places, the conclu- 
sions are given briefly in this note. 
1st, From the means of 9 years’ observations (1841-9) at Greenwich, the atmospheric pressure is a maximum from May to Septem- 
ber, the secondary minimum seen between these months at Makerstoun being wholly wanting; it is also a maximum in December, and, 
unlike the Makerstoun result, the mean for December is the greatest ; it is a minimum in October and November, as at Makerstoun ; 
and it is a minimum again in April, about two months after the corresponding minimum for Makerstoun. 
2d, When we compare the Greenwich monthly means for the 8 years 1842-9 with the Makerstoun monthly means for the same 
years, both being reduced to the level of the sea, and to 32° Fahrenheit, we find the barometer at Greenwich higher than at Maker- 
stoun in each month, and for the whole period as follows :— 
Jan. Feb. March. April. May. June. July. Aug. Sept. Oct. Noy. Dec. Year. 
in. in. in. in. in. in, in, in. in, in. in, in. in, 
0167 0133 0-097 0-031 0:014 © 0-087 0-114 0097 =—-0:035 0-112 0140 0-146 0:098 
Whence it appears that for the same (sea) level, the barometer, on the average of 8 years, is one-tenth of an inch lower at Maker- 
stoun than at Greenwich, 4° 6’ farther south; and that this difference of pressure varies with the month. The excess of the atmos- 
pheric pressure at Greenwich over that at Makerstoun is a principal maximum in January, the coldest month; and it is a maximum 
again in July, the hottest month ; it isa minimum in April and May, and again in September. It may be remarked, with reference 
to this curious result, that the positions of Greenwich and Makerstoun are much alike; nearly on the same meridian, and nearly 
equi-distant from the eastern coast of the island. There is no doubt that the greater proximity of Greenwich to the Continent has 
an effect upon its temperature, the mean temperature of Greenwich being only 2° kjgher than that of Makerstoun in winter, while 
it is 5° higher in summer. 
8d, From the mean of 8 years the atmospheric pressure at Greenwich is 0-098 inch greater than at Makerstoun, but the excess 
is by no means constant for each year; the excesses for each year are,— 
1842. 1843. 1844. ” 1845. 1846. 1847, 1848. ' 1849. 
in. in, in, in. in. in. in. in. 
0-119 0-096 0070 0-096 0-102 0-116 0:104 0-079 
The excess varies as much as half its mean value, and appears, on the whole, greatest in the hottest years and least in the coldest. 
4th, The following coincidences may be mentioned. The epochs of the annual law of mean atmospheric pressure (especially those 
for Greenwich) are nearly the same as for the annual law of magnetic declination (No. 9); and the law of differences of pressure 
for the two places has nearly the same epochs as the annual law for the magnetic force (No. 136). 
MAG. AND MET. oBs. 1845 anp 1846. z 
