PRESSURE OF Dry AIR. 283 
The mean meteorological seasons, for which June, July, and August constitute summer. 
The meteorological seasons for the year 1848, for which July, August, and September constitute summer. 
The astronomical seasons, for which May, June, and J uly constitute summer. 
Seasons. Mean Meteorological. Meteorological for 1843. Astronomical. 
in. in. in. 
Spring, . . 29-338 29-299 29-329 
Summer, A ‘ *266 -350 “297 
Autumn, : : 3138 356 316 
Winter, : 5 378 -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. 22h, Oh, 
in. in. in. in. in. in, in. in. in. in. | in. 
January 29-121 | 29-140 | 29-153 | 29-151 | 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 || -092 
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 | -561 054 090 
October -169 “173 +168 “151 -149 “135 -137 “148 | -154 7156 | -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 ease of the total atmospheric pressure. 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. 
