OBSERVED AND AN EMPIRICAL DEW-POINT. 
143 
not come into use until many years after the invention of Daniell’s instrument, and 
from the want of correct tables, based on an accurate knowledge of the relation ex- 
isting between the air and evaporation temperatures and the temperature of the dew- 
point, they were, until recently, of comparatively little service to the scientific ob- 
server. 
When the magnetic and meteorological observatory was, at the suggestion of the 
Royal Society, established at Greenwich, two-hourly observations of the dry- and wet- 
bulb thermometers began to be taken, whilst the dew-point from Daniell’s hygro- 
meter was recorded every six hours, simultaneously with the temperature of the dry 
and wet bulbs ; and these readings have been continued up to the present time. 
Such frequent observations, made by careful and competent observers with the best 
instruments, could not fail in time to afford the requisite data for the computation of 
an empirical dew-point, at all known temperatures of the air. The volume of Green- 
wich Magnetical and Meteorological Observations (copies of which the Astronomer 
Royal is kind enough to send me) for 1843, contains a complete discussion of the 
results down to 1845, from which the following factors were deduced: — 
When the' 
^the difference between the" 
'gives the difference be- 
tempera- 
ture of 
the air is^ 
^ below 24°< 
temperature of evaporation 
and the temperature of the 
air multiplied by 
. 8°*5 < 
tween the temperature 
of the air and the tempe- 
rature of the dew-point. 
Between 
24 and 25 
35 
h 
35 
Between 
25 and 26 
55 
6-4 
55 
Between 
26 and 27 
55 
6-1 
35 
Between 
27 and 28 
55 
5-9 
53 
Between 
28 and 29 
55 
57 
35 
Between 
29 and 30 
33 
5-0 
55 
Between 
30 and 31 
55 
4*6 
53 
Between 
31 and 32 
33 
3-6 
53 
Between 
32 and 33 
55 
3-1 
55 
Between 
33 and 34 
35 
2-8 
53 
Between 
34 and 35 
55 
2-6 
35 
Between 
35 and 40 
35 
2-5 
35 
Between 
40 and 45 
53 
2-3 
35 
Between 
45 and 50 
33 
2-1 
35 
Between 
50 and 55 
35 
20 
33 
Between 
55 and 60 
33 
]-8 
35 
Between 
60 and 65 
35 
1-8 
35 
Between 
65 and 70 
33 
17 
55 
above 70 
35 
1-5 
55 
From these data, Mr. Glaisher, of the Royal Observatory, in the year 1847, cal- 
