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328
THE HONGKONG GOVERNMENT GAZETTE, 24TH APRIL, 1886.
GOVERNMENT NOTIFICATION.. No. 141.
The following Report of the Director of the Observatory, on Results of Barometric Observations made during 25 Years in Hongkong, is published for general information.
By Command,
Colonial Secretary's Office, Hongkong, 24th April, 1886.
FREDERICK STEWART,
Acting Colonial Secretary
RESULTS OF BAROMETRIC OBSERVATIONS MADE DURING 25 YEARS IN
HONGKONG.
The following table exhibits the monthly mean barometric pressure as observed at the Harbour Office, 1861-1883 inclusive, and at the Observatory, 1884-1885 inclusive. Corrections have beer applied for index-errors of the barometers and the readings have been reduced to 32° Fahrenheit at Mean Sea Level but not to gravity at 45° latitude.
Similar observations have been, for many years, made at the Hospital, of which the monthly means were also published in the Gazette, but owing to an erroneous manner of recording these observation: they are not reliable.
The barometer was changed at the Harbour Office at the end of 1863 and in 1874. The index errors of those barometers were determined at the Observatory but I have not been able to learn anything about the one in use before 1863. No regular account was, however, kept of alterations in or repair! done to the instruments, and the following results are in consequence rather uncertain.
The hours of the observations adopted were 6 a., Noon, and 6 p. from January, 1861 to July, 187 inclusive; 10 a., Noon, and 4 p. from August, 1873 to July, 1876 inclusive; 9 a., Noon, and 3 p. from August, 1876 to December, 1880 inclusive; 10 a. and 4 p. from January, 1881 to December, 188 inclusive; and hourly readings in 1884 and 1885. No correction for barometric tide was called for as moreover marine barometers in which the tide is diminished, were used at the Harbour Office.
Most of the means were taken in 1883 and early in 1884.
The following formula for calculating the height of the barometer, B, in inches, at any time of th year expressed in degrees assuming the whole year to correspond to 360°, counting from the middle of December, was obtained from the monthly means:
B=29.953+0.236 cos. (p-31°) +0.012 cos. (26—352°).
The epochs of the highest and lowest barometer are obtained by differentiation of this equation The epochs of quickest change by double differention.
Assuming : We obtain :
Substituting:
B-B,
1
B, cos. (-e) + B2 cos.. (2-2).
dB
B, sin. (-)- 2 B sin. (2 - €1⁄2) — 0
2
dQ
B.
» %
P
0.
tg. († — § €) = X, tg. (e, -a and 4
} )
B1 sec. (€,- €) = b. we obtain: X*— 2a X3 + (1 + a2 —b2) X2 2 a X + a2 This equation has four roots, positive or negative according as a is positive or negative, but in either case two are imaginary.
Practically, however, the maximum and the minimum are obtained by trial. Thus the maximun 30.194 was found to occur about the 9th of January, and the minimum 29.719 about the 23rd July.
The monthly means reduced to 45° latitude are exhibited below and compared with the monthly means issued from Zi-ka-wei, near Shanghai, which are based on observations made from 1873 to 1884 inclusive. The latter were reduced to sea level and to 45° latitude :
Month.
January,
February,.
March, April, May,. June, July, August, September,
October,
•
November,
December,
Year,
Zi-ka-wei.
30.337
Hongkong.
30.116
Difference.
+ 0.221
.266
.069
.197
30.172
30.007
.165
29.995
29.892
.103
.852
.788
.064
.747
.708
.039
.682
.677
.005
.726
.680
.046
29.893
.762
.131
30.103
29.928
.175
.237
30.052
.185
30.307
30.102
+ 0.205
30.026
29.898
+ 0.128
It appears that the mean height of the barometer is greater in Zi-ka-wei than in Hongkong during every month of the year, but that the difference increases from July, when the heights are on an average nearly equal, till January.
W. DOBERCK, Government Astronomer.
Hongkong Observatory, 22nd April, 1886.