APPLIED TO METEOROLOGICAL OBSERVATIONS. 
135 
In every month the largest number in this table to which the plus sign is affixed, 
indicates the time of minimum temperature of the dew-point; and the largest number 
to which the minus sign is affixed indicates the time of maximum temperature of the 
dew-point. The laws which we may deduce from the numbers in this table are as 
follows : — 
At Greenwich the temperature of the dew-point attains its minimum in the first 
two months, and in the last two months of the year, some hours before the time of 
sunrise ; as the sun rises above the horizon and approaches the meridian, evaporation 
increases, and the air constantly receives a greater quantity of vapour, consequently 
the temperature of the dew-point increases till, at about the time of the maximum 
temperature of the air, the maximum temperature of the dew-point takes place. In 
summertime the minimum temperature of the dew-point attains its minimum a little 
before sunrise, and its maximum at about noon. In winter time, after having at- 
tained its maximum, the temperature of the dew-point decreases very regularly till 
next morning. In summer time the value remains very nearly at its maximum value 
till after the temperature of the air begins to decline ; it then very regularly decreases 
as before, till the following morning. 
The fact of the almost stationary temperature of the dew-point during the early 
afternoon hours in summer is important, and will be referred to again presently. 
Table IX. — Showing the corrections to be applied to the monthly mean elastic force of 
vapour at the height of four feet above the soil at any hour, to deduce the true mean 
elastic force of vapour for the month from the observations taken at that hour. 
Local mean 
time. 
January. 
February. 
March. 
April. 
May. 
June. 
July. 
August. 
Sept. 
October. 
Nov. 
Dec. 
in. 
in. 
in. 
in. 
in. 
in. 
in. 
in. 
in. 
in. 
in. 
in. 
Midnight. 
+ 0-006 
+ 0-006 
+ 0-008 
+ 0-017 
+ 0-026 
+ 0-031 
+ 0-028 
+ 0-025 
+ 0-024 
+ 0-018 
+ 0-010 
+ 
0-009 
1 A.M. 
+ 0-011 
+ 0-008 
+ 0-010 
+ 0-021 
+ 0-028 
+ 0-037 
+ 0-031 
+ 0-031 
+ 0-030 
+ 0-020 
+ 0-012 
0-010 
2 
+ 0-015 
+ 0-010 
+ 0-011 
+ 0-024 
+ 0-031 
+ 0-043 
+ 0-036 
+ 0-035 
+ 0-035 
+ 0-021 
+ 0 015 
+ 
0-010 
3 
+ 0-015 
+ 0-011 
+ 0-013 
+ 0-027 
+ 0-032 
+ 0 048 
+ 0-038 
+ 0-039 
+ 0-037 
+ 0-023 
+ 0-017 
+ 
0-011 
4 
+ 0-015 
+ 0-013 
+ 0-015 
+ 0-029 
+ 0-031 
+ 0-047 
+ 0-037 
+ 0-040 
+ 0-040 
+ 0-025 
+ 0-019 
+ 
0-011 
5 
+ 0-015 
+ 0-014 
+ 0-016 
+ 0-029 
+ 0-027 
+ 0-037 
+ 0-031 
+ 0-038 
+ 0-040 
+ 0-023 
+ 0-021 
+ 
0-0)1 
6 
+ 0-014 
+ 0-015 
+ 0-016 
+ 0-025 
+ 0-019 
+ 0-022 
+ 0-019 
+ 0-029 
+ 0-033 
+ 0-021 
+ 0-021 
+ 
0-010 
7 
+ 0-013 
+ 0-014 
+ 0-014 
+ 0-016 
+ 0-007 
+ 0-008 
+ 0-007 
+ 0-014 
+ 0-022 
+ 0-018 
+ 0-018 
+ 
0-009 
8 
+ 0-010 
+ 0-010 
+ 0-010 
+ 0-005 
-0-005 
— 0-004 
— 0-004 
0-000 
+ 0-010 
+ 0-011 
+ 0-012 
+ 
0-007 
9 
+ 0-007 
+ 0-006 
+ 0-005 
+ 0-005 
-0016 
-0-015 
-0-014 
-0-012 
— 0-005 
+ 0-005 
+ 0-005 
+ 
0-005 
10 
+ 0-002 
0-000 
— 0-003 
— 0-013 
— 0-024 
— 0-027 
— 0-019 
-0-021 
-0-019 
-0-005 
— 0-004 
+ 
0-001 
11 
— 0-004 
-0-005 
-0-007 
-0-020 
— 0-028 
— 0-036 
— 0-025 
-0 027 
— 0-027 
-0-009 
— 0-010 
0-004 
Noon. 
-0-007 
-0-009 
— 0-012 
— 0-026 
— 0-030 
-0-042 
— 0-029 
-0-030 
-0-030 
-0-015 
—0-017 
— 
0-007 
1 P.M. 
— 0-008 
— 0-013 
— 0-013 
— 0-027 
-0-030 
-0-045 
-0-033 
-0-032 
-0-030 
-0-018 
— 0-019 
— 
0-008 
2 
-0-007 
—0-015 
— 0-013 
— 0-027 
-0-028 
-0-043 
-0-034 
— 0-034 
— 0-029 
-0-017 
-0-020 
— 
0-008 
3 
-0-007 
-0-012 
-0-012 
-0-025 
— 0-026 
-0-039 
-0-033 
— 0-031 
-0-027 
-0-014 
— 0-016 
— 
0-008 
4 
— 0-007 
— 0-010 
-0-010 
— 0-020 
— 0-021 
-0-035 
— 0-028 
-0-027 
-0-021 
-0-009 
-0-010 
— 
0-007 
6 
— 0-004 
-0-006 
— 0-006 
-0-014 
-0-015 
-0-025 
— 0-021 
-0-020 
-0-017 
— 0-006 
-0-005 
— 
0-005 
6 
-0-002 
-0-004 
—0-002 
—0-006 
— 0-010 
-0-017 
— 0-016 
-0-015 
-0-010 
— 0-004 
0-000 
— 
0-003 
7 
— 0-001 
— 0-001 
+ 0-002 
+ 0-001 
-0-004 
-0-007 
— 0-007 
-0-006 
-0-003 
+ 0-003 
+ 0-004 
— 
0-001 
8 
0-000 
+ 0-001 
+ 0-004 
+ 0-005 
+ 0-005 
+ 0-005 
+ 0-004 
+ 0-004 
+ 0-004 
+ 0-005 
+ 0-006 
+ 
0-001 
9 
0-000 
+ 0-003 
+ 0-005 
+ 0-007 
+ 0-013 
+ 0-015 
+ 0-010 
+ 0-010 
+ 0-008 
+ 0-008 
+ 0-008 
+ 
0-004 
10 
+ 0-001 
+ 0-004 
+ 0-007 
+ 0-010 
+ 0-017 
+ 0-023 
+ 0-017 
+ 0-015 
+ 0-013 
+ 0-011 
+ 0-009 
+ 
0-005 
11 
+ 0-002 
+ 0-005 
+ 0-008 
+ 0-014 
+ 0-022 
+ 0-029 
+ 0-024 
+ 0-020 
+ 0-018 
+ 0-014 
+ 0-010 
+ 0-006 
The sign + denotes that the number is to be added, and the sign — denotes that 
the number is to be subtracted. 
