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Instructions for keeping the Meteorological Log.
Observations should be made every four hours, and the latitude and longitude of the vessel should be entered at each observation.
If convenient an observation should be made in or near Hongkong.
Observations are required between 10° South and 45° North latitude and between Singapore and 180° E of Greenwich,
When a mercurial barometer is read the thermometer attached to it is also entered.
When an aneroid is read no thermometer reading should be entered.
Force of wind is given from 0 to 12, and weather in Beaufort's initials.
For further particulars the "Instructions for making Meteorological Observations prepared for use in China" published in 1883 by the writer, may be consulted.
The forms are forwarded free through British Post Offices in China if addressed on service. They should be posted as soon as convenient after the vessel enters Hongkong harbour. Vessels bound for London from ports in the China Sea should post the forms in Singapore.
In 1896 the total number of days' observations made on board 325 ships and forwarded to this Observatory was 18,541 (counting separately those made on board different ships on the same day).
The surest of all warnings is furnished by the standard barometer on shore and the compensated aneroid on board ship; you are all right if you can put your vessel on the tack that will keep your barometer rising. But in order to understand the indicatious of the barometer you will have to keep a regular meteorological register. The master of a vessel who does not look at his aneroid till he is in a typhoon, does not derive half the benefits from his observations that he would have enjoyed had he watched it beforehand. He might perhaps have avoided the weather he is now experiencing, or even have benefited by the favourable winds and sailed round the typhoon. No doubt, the time is approaching when underwriters will stipulate that the indications of an aneroid or a marine barometer must be regularly registered on board a vessel insured by them.
On the other hand it would not be fair to ask the mariners to keep complete meteorological records, such as are kept in the lighthouses out here. Some seamen have a taste for this kind of work and make very useful and fairly accurate observations, but, for instance, the readings of dry and damp bulb thermometers taken on many vessels are of very little use.
The tube of the marine barometer has to be so much contracted to stand the incessant pumping and danger of breakage, that the instrument is sluggish and often reads half an inch or more too high near the centre of a typhoon. Some cheap wooden barometers cannot be registered below a certain height, the cistern being too small to hold the mercury that comes out of the tube and there is the great objection to wooden mercurial barometers that the readings cannot be accurately reduced to freezing point and the temperature correction is larger than in case of instruments made of brass. Of course, some cheap aneroids are no better, and even a first class compensated instrument requires to be thoroughly verified, as the scale is never quite correct and the readings depend somewhat upon the temperature and in a manner different for each single instrument, so that general tables for correcting to freezing point are not available, but they act more quickly than the marine barometer, and for use on board ship the instrument that is quickest in its indications must be preferred. The objection to the use of the anëroid is founded on the fact that its index-correction changes gradually; but then this can be determined and allowed for by reading it off as often as the vessel euters a port, such as Hongkong, where correct ineteorological observations are constantly being made.
The best hours for making observations are 4 a.m., 8 a.m., etc., up to midnight inclusive. From 4 a.m. to 10 a.m. the barometer is rising, falling from 10 a.m. to 4 p.m., rising from 4 p.m. to 10 p.m., and falling from 10 p.m. to 4 a.m. It reads highest at 10 a.m. and lowest at 4 p.m. The daily variation is twice as great in midwinter as it is in midsummer. During the approach of a typhoon this regular daily variation may be masked, but it goes on all the same and must be taken into account when the barometer begins to fall before a typhoon. Thus if it has fallen a certain amount between 10 a.m. and 4 p.m. you must subtract the normal descent between these hours in order to know how much of the fall is due to the approach of the typhoon, and if it were between 4 p.. and 10 p.m. that it fell, you must add the normal rise for the same purpose.
§4.-ON THE DIFFERENT CLASSES OF TYPHOONS AND THE SEASONS
OF THE YEAR IN WHICH THEY APPEAR.
In 1886 I expressed the hope that it might be possible to construct average paths of all the different varieties of typhoons, when a couple of hundred tracks were available. This has just been effected on the basis of 244 typhoons, registered during the past 13 years, or on an average 19 typhoons per year. They are distributed among the different months of the year as follows: January 1, February 0, March 1, April 4 (2 per cent), May 10 (4 p.c.), June 24 (10 p.c.), July 45 (19 p.c.), August 43 (18 p.c.), September 57 (23 p c.), October 31 (13 p.c.), November 22 (9 p.c.), December 6 (2 p.c.).