272 



Abstracts of the Makerstoun Observations, 1843. 



TABLE XI. — Hourly Means of the Pressure of Aqueous Vapour for each Month, as deduced from 

 Tables III. and VIII., with the Means for the Quarters and Year 1843. 



Period. 



IS"'. 



^Oh. 



22''. 



0"^. 



2h. 



4'>. 



6h. 



81'. 



lOh. 



Mean. 



Range. 





ill. 



in. 



in. 



in. 



in. 



in. 



in. 



in. 



in. 



I 



in. 



in. 



January 



0-212 



213 



0210 



0-214 



0-219 



0-221 



0-222 



0-211 



0^208 



0213 



0014 



February 



•178 



177 



•176 



•180 



•184 



■185 



•183 



-179 



•183 



1 ^180 



■009 



March 



•201 



207 



■221 



•236 



•239 



•235 



•231 



•223 



•218 



•220 



■038 



April 



•231 



248 



•263 



•260 



•265 



•261 



•259 



■256 



•244 



•250 



■034 



May 



■264 



280 



•287 



•292 



•289 



•293 



•285 



•284 



.279 



•281 



•029 



June 



•308 



317 



■321 



•322 



•336 



•332 



•333 



•318 



•312 



•319 



•028 



July 



•364 



393 



•398 



•404 



•407 



•395 



•403 



•392 



•378 



•387 



•043 



August 



•355 



392 



■423 



•444 



•443 



•435 



•437 



•433 



•415 



•411 



•089 



September 



•324 



360 



•396 



•414 



•417 



•405 



•414 



•402 



•380 



• 381 



•093 



October 



•233 



241 



•251 



•264 



•258 



•259 



•252 



•243 



•237 



•245 



•031 



November 



•230 



235 



•239 



■242 



•249 



•239 



•232 



•232 



•229 



235 



•020 



December 



•273 



273 



•275 



■288 



•289 



•285 



•277 



•273 



•273 



•277 



•016 



Spring 



•232 



245 



•257 



•263 



•264 



•263 



•258 



•254 



•247 



•250 



•032 



Summer 



•342 



367 



•381 



•390 



•395 



•387 



•391 



•381 



•368 



•372 



•053 



Autumn 



•262 



279 



•295 



•307 



•308 



•301 



•299 



•292 



•282 



•287 



•046 



Winter 



•221 



221 



•221 



•228 



•231 



•230 



•228 



•221 



•222 



•227 



•010 



The Year 



•264 



278 



•288 



•296 



•299 



•295 



•294 



•287 



•279 



•283 



•035 



The previous Table has been formed from Tables III. and VIII. by means of the formula given after 

 Table IX. 



The Diurnal Variation of the Pressure of Aqueous Vapour also follows somewhat nearly the march of 

 temperature of the air ; there are, however, some apparent irregularities in the progression of the former that 

 neither appear in the latter nor in that of the temperature of evaporation. In January, the maximum of pres- 

 sure occurs about 4 p.m., in February, about 3 p.m., while, in the other months, it occurs nearer 1 p.m. than 

 any other hour. In some of the months there are one or more secondary minima ; some of these are so marked 

 as to render it probable that they are not accidental. In the month of April, a secondary minimum occurs 

 aboiit 11 A.M. ; in May and October, about 1 p.m. ; and in June, July, August, and September, about 3 p.m. ; the 

 maxima occurring about two hours before and after the minima. The minima are most distinctly marked in 

 July, August, and September. The occurrence of minima, as here indicated, is, perhaps, what might have been 

 expected fi'om the non-coincidence of the periods of maxima for the temperature of the air and the temperature 

 of evaporation. Thus, taking the most marked case, the month of August, the temperature of the air and the 

 temperature of evaporation go on increasing together till a little after 1 p.m. ; the temperature of evaporation then 

 commences falliag ; the temperature of the air, however, increases for nearly three-quarters of an hour after 

 this. The increasing pressure of aqueous vapour will, therefore, evidently receive a sudden check at the time 

 of the maximum temperature of evaporation, and it will diminish rapidly while the temperature of the air and 

 of evaporation are moving in opposite directions. When, however, the temperature of the air commences falling 

 also, the pressure of vapour will diminish less rapidly, until the falling temperature of the air makes up for its 

 lost time and gains ground on the falling temperature of evaporation, thus producing a second maximum of pres- 

 sure ; after this they diminish together, according to nearly the same law as they increased in the morning. The 

 occurrence of the maximum temperature of evaporation later than that of the air will evidently produce a 

 minimum before the temperature of evaporation attains its maximum.* 



* The afternoon secondary maximum and minimum seemed to me, at first, due to a local cause, namely, the action of the sun on 

 the soil near the thermometers when it approaches the prime vertical, producing in this way an abnormal state of the atmosphere 

 near the thermometers. I have, however, been induced to reject this hypothesis for the following reasons : — With a similar amount 

 of sunshine, a similar action should be visible in the morning, but there is none visible ; the effect should be most distinct near mid- 

 summer, whereas it is most evident in August and September ; it should be as well marked in March, April, and May, but it is not 

 evident at all in these months. The comparative amounts of sunshine for the year 1843 can only be estimated from the observed 

 surface of cloud, and this differs little before 7 a.m. and after 5 p.m., but it is evident that the quantity of vapour may be connected 

 with the surface of cloud by other than local considerations, as will be seen on examination of the tables for the surface of cloud. 

 Finally, the minimum occurs at V^ p.m. in May and October, and at IV^ a.m. in April, when the cause supposed could not operate. 



