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18. Magnetic Observations made at the Hongkong Observatory in the years 1890 and 1891. The observations, of declination and horizontal force were made with the unifilar magnetometer, Elliott Brothers, No. 55, and the dips were observed with dip-circle, Dover No. 71. The observations were discontinued for some time while there were no assistants in the. Observatory, but that is not likely to occur again.
The methods adopted in making the observations and in determining and applying the corrections are explained in Appendix G of Observations and Researches made in 1885; "On the verification of the unifilar magnetometer, Elliott Brothers, No. 55," the value of log 7 K was 3.45020 at 29° cent. in 1890, and 3.44949 at 27° in 1891. The value of P was +7.292 in 1890, and + 5.854 in 1891. These constants are now determined separately at each observation. The mean value of the magnetic moment of the vibrating needle was in 1890, 0.47120 in English units and 615.19 in C.G.S. units, and in 1891, 0.46543 in English units and 607.66 in C.G.S. units.
The times of vibration exhibited in the table are each derived from 12 observations of the time occupied by the magnet in making 100 vibrations, corrections having been applied for rate of chro- nometer and are of vibration.
The observations of horizontal force are expressed in C. G. S. units (one centimeter, one gramme, one second), but the monthly synopsis exhibits X, the horizontal, as well as Y, the vertical, and the total forces, which have been computed by aid of the observed dips, and their values are also given in English units (one foot, one grain, one second) and in Gauss's units (one millimeter, one milligram, one second.)
19. At the Observatory the cisterns of the barograph and standard barometer are placed 109 feet above mean sea level. The bulbs of the thermometers are rotated 108 feet above mean sea level and 4 feet above the grass. The solar radiation thermometer is placed at the same height. The rim of the rain-gauge is 105 feet above mean sea level and 21 inches above the ground. At Victoria Peak, the instruments, except the rain-gauge are placed in the look-out. The cistern of the barometer is 1814 feet above sea level. The bulbs of the thermometers are 4 feet above the floor except the maximum thermometer which is a few inches higher. The rim of the rain-gauge is 8 inches in diameter and one foot above the ground.
20. The monthly Weather Reports are arranged as follows :—
Table I. exhibits the hourly readings of the barometer reduced to freezing point of water, but not to sea level, as measured (at two minutes to the hour named) from the barograms.
Tables II. and III. exhibit the hourly readings of the temperature of the air, and of the temperature of evaporation as determined by aid of rotating thermometers. Table II. exhibits also the extreme temperatures during. the day reduced to rotating thermometer. Table III. exhibits also the solar radiation (black bulb in vacuo) maximum temperature reduced to the Kew arbitrary standard.
Table IV. exhibits the mean relative humidity in percentage of saturation and mean tension of water vapour present in the air expressed in inches of mercury for every hour in the day and for every day in the month calculated by aid of Blandford's tables from the data in Tables II. and III.
Table V. exhibits the duration of sunshine expressed in hours from half an hour before to half an hour after the hour (true time) named.
Table VI. exhibits the amount of rain in inches registered from half an hour before to half an hour after the hour nained. It exhibits also the duration of rain estimated at the Observatory. The rainfall at Victoria Peak is measured at 10 A. and entered to preceding day, but it is evident that the observer does not attend to the gauge every day.
Table VII. exhibits the velocity of the wind in miles and its direction in points (0-31). The velocity of the wind is measured from half an hour before to half an hour after the hour named, but the direction is read off at the hour.
Table VIII. exhibits for every hour in the day, the mean velocity of the wind reduced to 4 as well as to 2 directions, according to strictly accurate formulæ, and also the mean direction of the wind.
Table IX. exhibits the readings of the barometer reduced to freezing point of water, but not to sea level, and of the thermometers, and the observations of the direction (to two points) and force (0-12) of the wind at Victoria Peak.
Table X. exhibits the amount (0-10), name and direction whence coming of the clouds. Where the names of upper and lower clouds are given, but only one direction, this refers to the lower clouds.
21. The following annual weather report for 1891, is arranged as follows:-
Table VII. exhibits the mean values for the year (or hourly excess above this) obtained from the means given in the monthly reports. The mean hourly intensity of rain is obtained from Table VI. of the monthly reports in connection with Table VIII of this report. The total amount of rain was 117.120 at the Observatory, and 128.66 at the Peak. The total duration registered at the Observatory was 917 hours. There fell at least 0.01 inch of rain, on 154 days at the Observatory.