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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 VIIL., with the Means for the Quarters and Year 1843. 
Period. 184, 202. 22h. Ob, 2h. 4h, 6h, | 8h, 10%, Mean. || Range. 
I} | 
P na ; : F : 5 er ; | 
im.) |. an. in. in. in. in. in, | in in. in. in. 
January | 0-212 | 0-213 | 0-210 | 0-214 | 0-219 | 0-221 | 0-222 | 0-211 | 0-208 | 0-213 || 0-014 
February -178 | -177 | -176| -180 | -184 | -185 | -183 | -179 | -183 |) -180 || -009 
March -201 | -207 | -221 | -236 | -239 | -2385 | -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 
about 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 from 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 falling ; 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 1" p.m. in May and October, and at 11" a.m. in April, when the cause supposed could not operate. 
