Pressure of Dry Air. 
283 
The mean meteorological seasons, for which June, July, and August constitute summer. 
The meteorological seasons for the year 1843, for which July, August, and September constitute summer. 
The astronomical seasons, for which May, June, and July constitute summer. 
Seasons. 
Mean Meteorological. 
Meteorological for 1843. 
Astronomical. 
Spring, 
in. 
29-338 
in. 
29-292 
in. 
29-329 
Summer, 
•266 
•350 
•297 
Autumn, 
•313 
•356 
•316 
Winter, 
•378 
•297 
•353 
Range of Means, 
0-112 
0-064 
0-056 
For the mean meteorological and astronomical seasons, the dry air is greatest in Winter and least in 
Summer ; but the meteorological seasons for 1843 give the pressure greatest in Autumn and least in Spring, 
and do not shew any distinct connexion between the pressure and temperature. Such a connexion, indeed, is 
not perceptible in the monthly means ; the high value of the winter means is due to the high pressure in 
December, a month, in as far as temperature is concerned, more autumnal than hibernal. 
The mean pressure of the dry air for 1843 = 29-324 in. 
TABLE XXI. — Hourly Means of the Pressure of Dry Air for each Month and Quarter of 1843. 
Period. 
18\ 
20". 
22 b . 
0 b . 
2 h . 
-l 1 '. 
6''. 
8 h . 
10K 
Mean. 
Range. 
in. 
in. 
in. 
in. 
in. 
in. 
in. 
in. 
in. 
in. 
in. 1 
January 
29-121 
29-140 
29153 
29151 
29-134 
29-136 
29-136 
29-156 
29-166 
29-144 ] 
0-045 
February 
•296 
•305 
•325 
•331 
■314 
•308 
•318 
•336 
•337 
•319 
•041 
March 
•446 
•455 
•448 
•434 
■423 
•421 
•427 
•447 
•454 
•442 
•034 
April 
•249 
•236 
■223 
•224 
•212 
•211 
■218 
•238 
•263 
•237 
•052 
May 
•369 
•356 
•347 
•336 
•330 
•313 
■313 
•323 
•331 
•339 
•056 
June 
•306 
•304 
•303 
•305 
•288 
•284 
•275 
•295 
•310 
•300 
• 035 ] 
July 
•277 
•248 
•243 
•238 
•226 
•234 
-.219 
•235 
. -254 
■248 
•058 
August 
•307 
•277 
•245 
•216 
•209 
•206 
•198 
•212 
•242 
■245 
•109 
September 
•602 
•581 
•553 
•530 
•516 
•521 
•512 
•533 
■561 
•554 
•090 
October 
•169 
•173 
•168 
•151 
•149 
•135 
•137 
•148 
•154 
•156 
•038 
November 
•201 
■206 
•221 
•228 
•227 
•242 
•261 
•267 
•274 
• 236 
•073 
December 
•689 
•689 
•700 
•689 
•670 
•668 
•679 
•684 
•688 
•685 
•032 
Spring 
•355 
•349 
•339 
•331 
■322 
•315 
•320 
•336 
•349 
•340 
•040 
Summer 
•297 
•277 
■263 
•253 
•241 
■242 
•231 
•247 
•269 
■265 
•066 
Autumn 
•324 
•320 
•314 
•303 
■297 
•299 
•304 
• 316 
■330 
•315 
•033 
Winter 
•369 
•378 
•392 
•390 
•372 
•371 
•377 
■ 392 
•396 
•382 
•027 
The Year 
•336 
•331 
•328 
•320 
•309 
•307 
•308 
•323 
•337 
•326 
•030 
This Table has been formed by subtracting Table XI. from Table XVIII. 
Diurnal Variation of the Pressure of Dry Air. — An examination of the monthly means will shew that 
they may be separated into two classes, namely, months in which two maxima and minima are visible, and 
months in which it is probable that only one maximum and minimum occur ; the same division was noticed in 
the case of the total atmospheric pi'essure. One month, September, which shews a double maximum and 
minimum of total pressure, shews only one of each for the dry air. The months in which double maxima and 
minima evidently occur are January, February, March, June, November, and December. A secondary and 
apparently accidental maximum occurs at 3 p.m. in some months. 
On the whole, the Winter months present two and the Summer months only one maximum and minimum 
this is also evident in the means of groups. 
