Annual Variations foe the Atmospheric Pressure. 



xci 



190. Annual Variation 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. 



Nov. 



Dec. 



in. 



in. 



in. 



in. 



in. 



in. 



in. 



in. 



in. 



in. 



in. 



in. 



0-043 



0-045 



0-037 



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 meanr 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 more certain ; these means are as follow : — 



Prefix I Jan. — Feb. — March — April — May — June — July — Aug. — Sept. — Oct. — Nov. — Dec. — Jan. 



29 in. I -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. 



in. in. in. in. 



29-561 29-660 29-669 29-569 



* 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 •. — 



Oct.-Mar. 



Apr.-Sept 



in. 



in. 



29-565 



29-665 



Jan. Feb. 



in. in. 



0-167 0133 



March. 



0'097 



April, May. June. 



0-031 



0-014 



0-087 



July. Aug. Sept. Oct. Nov. Dec. Year. 



in, in. in. in. in. in. in. 



0-114 0-097 0-035 0-112 0-140 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 is a 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 i1%temperature, the mean temperature of Greenwich being only 2° higher than that of Makerstoun in winter, while 

 it is 5° higher in summer. 



Zd, 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. 



0119 



0096 



0-070 



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, greates_t in the hottest years and least in the coldest. 



4«A, 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. DBS. 1845 AJfD 1846. Z 



