90 MR. T. HOPKINS ON IMPROVEMENT 
should not only be carefully tabulated, but graphically 
exhibited; and if it be found that as more vapour rises the 
barometer falls, and fo an extent proportioned to the quan- 
tity of vapour that is ascending, it may be taken as evidence 
that the vapour supplies the means of reducing atmospheric 
pressure. And if, in addition to this evidence, the tension 
of the vapour in the lower regions is found to be reduced, 
that fact may be considered a proof that the quantity of 
vapour in the air has become less because some of the quan- 
tity which had previously been there has been condensed ; 
and the barometer alone, by showing how much the atmo- 
sphere has been lightened, would be evidence of the total 
quantity of vapour that had been condensed. In tracing 
these facts it could hardly fail to be perceived that ascend- 
ing vapour is the important agent in reducing atmospheric 
pressure. 
From ten o’clock in the morning until four in the after- 
noon is generally the time when the barometer falls, and 
during this period the different heights of the wet and dry 
bulb thermometers show the energy of evaporation —when 
the tension shows the quantity that remains as vapour, — 
whilst the fall of the barometer indicates the quantity of 
heat liberated by condensation of vapour to warm and 
lighten the atmosphere. Such a system of recording 
atmospheric changes would present a tolerably clear view 
of the alterations that take place in the higher regions in 
the middle of the day, and the facts being presented in a 
tabular form any meteorologist would be enabled to com- 
pare the different phenomena, and to connect them with 
the working of known laws of nature. 
Hourly changes have been traced for the reason already 
given, viz., that we are better able to collect evidence of 
their nature than we are respecting those depending on 
the annual revolutions of the earth. But if we can prove 
what disturbs the atmosphere at particular hours so much 
