MR. HUDSON’S HOURLY OBSERVATIONS ON THE BAROMETER. 589 
2. The striking connection between the barometrical changes and the 
variations of temperature. 
3. The relation which appears to subsist between the variations before 
noon and those before midnight,—a great amount of variation before 
noon being followed, in the same mean day, by a corresponding small 
variation before midnight, and the contrary. 
Plate XXII. exhibits the simultaneous movements of the Water Barometer, 
the Standard Barometer, and the Mountain Barometer ; and points out,— 
4. The general accordance in the mean variations of three instruments, so 
dissimilar in principle and construction; and the remarkable nature 
of those differences which their simultaneous observation has elicited. 
5. The precession in time, by about an hour, of the mean motions of the 
Water Barometer over those of the Standard Barometer; and the 
precession, by the same interval, of the mean changes of this latter 
instrument, over those of the Mountain Barometer*.' 
Plate XXIII. exhibits a comparative view of a mean day’s observations 
in summer, with one in winter, after an interval of exactly half a year ; and 
displays,— 
6. The influence which the season of the year, or the temperature of such 
season, appears to exercise over the hours of maximum and minimum, 
and over the amount of the mean variations. The minimum and 
maximum of the morning are earlier, and those of the evening later, 
in summer than in winter: and the variations in summer are small 
about noon, and great about midnight; those in winter, the reverse. 
Plate XXIV. represents the mean result of the whole of the observations. 
The mean variations of the first five hours are referred to a general mean de¬ 
rived from all observations made continuously from 1 a.m. to midnight; those 
of the next two hours are referred to one derived from all observations made 
* I am not aware that any series of observations has before exhibited this singular result, and de¬ 
veloped the important influence which the diameter of the tube, and the nature of the fluid column 
exercise over the changes which the atmospheric pressure ought to produce in the barometer. Dr. 
Piiour has since informed me, that he has found a barometer made with sulphuric acid move with 
much greater freedom than the ordinary mercurial barometers,—a fact which he considers only to be 
explained by the greater mobility of the molecules of the liquid under these circumstances, and which 
strikingly corroborates this result of my observations. 
MDCCCXXXII. 4 G 
