350 REPORT—1846. 
For determining the influence of the rate of air-movement on the rate of 
cooling of the wet bulb, several processes have been employed, both in the 
open air and in the house. 
1. In the first place, choosing a day when the air was considered nearly 
calm, (%.e. moving so gently that only leaves with very flexible petioles were 
swayed by it,) the wet bulb was first suspended at res¢ in this gentle current 
(A); secondly, it was carried across it ( B) at the rate of 2°75 miles an hour; 
thirdly, it was swung across it at the rate of 6°18 miles an hour (C). The 
effects are recorded in the table below. 
Experiment of 22nd Aug. 1846. Temperature of air 75°, of evaporation 63° (€d=12). 
. B. Cc. 
Time of cooling 1° ...... 16° 12: 9° 
Reciprocals of these numbers, which represent the cooling powers of the air 
in the several conditions, appear below :— 
A. B. Cc. 
625 833 TLE 
and are obviously not in simple proportion to the velocity of the air-move- 
ment in the several cases. If we suppose the so-called ‘calm’ air to move 
two miles an hour (which the movement of smoke at the time seemed to in- 
dicate), the velocities, obtained by calculating the diagonals of the air-move- 
ments combined with the thermometer-movements, would be as under :— 
A. B. Cc. 
2:00 3°40 6:48, 
and their square roots, viz.— 
1-41 184 2:55 
approach nearly to the ratio of the cooling powers. 
2. Experiments of this kind however being unsusceptible of much preci- 
sion, recourse was had to railway movement in a ‘calm’ day. Temperature 
of air 70°, of evaporation 64° (d=6). Time of cooling 1°, at 3 inches from 
the carriage window, 14''; at 18 inches, 10"; at 24 inches, 9”. 
Again, on another occasion, temperature of air 69°5, of evaporation 64° 
(d=5'5). Time of cooling 1°, at 20 inches distance from the window, 10". 
In each of these cases the real velocity of the train was believed to be 
about 36 miles an hour. 
These experiments are sufficiently in accordance with those already dis- 
cussed to allow of our applying the formula a = VV toall; where T is 
the observed time of cooling 1° and C a constant peculiar to the instrument. 
By taking C = 300, and employing this value for the several experiments, 
the estimated and calculated results appear thus :-— 
ae gC velocity | Calculated velocity 
in miles. in miles, 
Calm air! deehetecectstee 2-00 2-44 
Thermometer carried ... 3°40 431 
Thermometer swung ... 6:48 771 
Railway movement...... 36°00 30°86 
Railway movement...... 36:00 29°75 
As all these experiments are complicated with the uncertain and variable 
influence of what is called ‘calm’ air, their accordance with one general for- 
mula appears quite as great as could be expected. The drag of air, which 
the unequal rates of cooling at different distances from the carriage indicate, 
