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, Jime, and July constitute summer. 



Seasons. 



Mean Meteorological. 



Spring, 



m. 



29-338 



Summer, 



•266 



Autumn, 



•313 



Winter, 



•378 



Range of Means, 



0-112 



Meteorological for 1843. 

 in. 



29^292 

 •350 

 ■356 

 •297 



0-064 



Astronomical, 

 in. 



29-329 

 •297 

 •316 

 •353 



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 pi-essure iu 

 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. 



18i>. 



20''. 



22''. 



0^ 



2^ 



41'. 



6''. i 



?!'. 



101'. M 



ean. 



Range. 





in. 



in. 



in. 



in. 



in. 



in. 



in. 



in. 



iu. 



in. 



in. 



January 



29-121 



29140 



29-153 



29^151 29 



134 



29^136 



29^136 29 



156 



29^1 66 29 



144 



0045 



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 oxamiuation of the monthly means will sliew that 

 tliey may be separated into two classes, namely, months iu 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 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. 



