82 MR. T. HOPKINS ON IMPROVEMENT 
Here we may perceive that at four o’clock in the 
morning, the barometer stood at 29°5831 inches, and 
that by nine in the morning it had risen to 29°6084, 
being a rise of 0°0253 of an inch. At the first named 
hour, the tension of vapour was 0°4420, and by nine it 
had risen to 0°5550, or 0°1130 of an inch, leaving for 
the separate pressure of the gases at the first hour 
29°1411 inches, from which it declined to 29-0534 at 
nine; so that the increase in the pressure of vapour 
must have counteracted the whole influence of increased 
temperature during the time, and also raised the baro- 
meter 0°0253 of an inch. The additional vapour pressure 
must have been a result of greater energy of evaporation, 
arising from the higher temperature sending more vapour 
into the air, and the increased evaporation is shown in 
the difference in the registrations of the wet and dry ther- 
mometers, as seen in the table above. This amounted 
to 2°01 at four o’clock, and increased to 5°81 by nine 
o'clock ; and we have seen that up to that hour enough 
vapour had been sent into the air to raise the barometer 
0:0253 of an inch, as well as to counteract the influence 
of the increased temperature on the gases. 
A change now took place, and after nine o’clock the 
barometer fell until five in the afternoon, when it was as 
low as 29-5588 inches; and here we naturally ask: What 
was, at this time, the influence of the aqueous vapour 
which had just before shown itself so powerful in producing 
a rise in the barometer? It would seem to follow as a 
necessary consequence that as a rise of that instrument 
was caused by an increase of vapour, a fall would be 
accompanied by a decrease, or at least that the tension 
of vapour would show a reduction when the barometer fell 
equal to its increase when that instrument rose. 
At nine o’clock, the time when the barometer began to 
fall, we have seen that evaporation was at 5°81, but 
