52.

(589.) D=58° 1′ 22′′; chron. fast for M. Gr. T. 20 m. 49 s.; long. 118° 31′ 15′′ W.

(590.) D=87° 42′ 50′′; chron. slow for M. Gr. T. 1 m. 24 s.; long. in 26° 7' W.

(591.) D=40° 52′ 43′′; chron. fast 1 m. 4s. for M. Gr. T.; long. 69° 8′ 45′′ W.

(592.) D=94° 5′ 38′′; chron. fast 57 s. for M. Gr. T.; long. 67° 27' 0" E.

(593.) D=68° 52′ 10′′; chron. fast 20 s. for M. Gr. T.; long. 178° 48′ 15′′ W.

(594) D= 101° 1′ 42′′; chron. slow 13 s. for M. Gr. T.; long. 1° 53' 15" W.

(595.) D=71° 38′ 20′′; chron. fast 41 s. for M. Gr. T; long. 144° 56' 15" W.

(596.) D=60° 17' 50"; chron. slow 30 s. for M. Gr. T.; long in 120° 37' 30" E.

(597.) D=76° 25′ 58′′; chron. fast 59 s. for M. Gr. T.; long. in 74° 11′ 0′′ E.

(598.) D=40° 22' 3"; chron. fast 19 s. for M. Gr. T.; long. in 177° 12′ 15′′ E.

(599.) D=96° 34′ 22′′; chron. fast 1 m. 41 s. for M. Gr. T.; long. in 156° 39′ 30′′ W.

(600.) D=94° 5' 26"; chron. fast 1 m. 23 s. for M. Gr. T.; long. in 67° 33′ E.

APPENDIX.

ON WEATHER FORECASTS AND STORMS.

IN the previous chapters of this work, there have been given such rules and directions as will be found sufficient to enable the mariner to find his place at sea, and to determine on what course he should steer, so as to reach any desired port whose latitude and longitude are known; and there the principles on which these rules and directions are founded, have also been given and demonstrated mathematically.

But in conducting a ship through the wide and pathless ocean, it is often found that the wind and waves are so directly opposed to the course desired to be steered, that the treatise would be defective unless it contained directions to the mariner, by which he may at least so far anticipate the kind of weather he is likely to encounter, and be prepared to adopt the wisest measures when actually overtaken by a storm.

In this chapter, then, our plan must be somewhat different from that followed in the previous chapters, for we can no longer be entirely guided by the unfailing principles of mathematical science, but must also call to our aid the established facts of meteorology to guide us in forecasting the

weather, and the experience of seafaring men, to enable us to ascertain, as far as possible, the laws which storms at sea are found in general to follow, and the best means of avoiding their disastrous consequences.

THE BAROMETER.

The principal instrument used in weather-casts is the barometer, which must also be accompanied by the thermometer; for the weight of the atmospheric air, which is shewn by the height of the barometer, depends in a great degree on its temperature, which is shewn by the thermometer, since heat expands and cold contracts the volume of the atmospheric air, as well as that of solid bodies. In predicting changes of weather, therefore, from the height of the barometer, and its rising and falling, the height of the thermometer must not be neglected, for, by such neglect, the results obtained would in most cases be erroneous, and might in some cases be the very opposite of those at which a skilful observer would arrive, by a proper balancing of the indications of the two instruments.

The barometer, in its simplest and best form, is a hollow tube of glass of uniform bore, and upwards of thirty inches in length, hermetically sealed at one end and open at the other. This tube being filled with mercury, and placed in a vertical position, with the closed end uppermost, and the lower open end inserted in a cup or cistern filled with mercury, the communication between the tube and cistern being made free, the mercury falls in the tube till its weight is equal to that of a column of atmospheric air, of the same diameter, reaching from the surface of the earth to the utmost limits of the atmosphere. The chief difference between this common barometer and the marine one is, that in the latter there is at one part of the glass tube a narrow contraction, through which the mercury can

only pass slowly, to prevent the oscillation of the mercury in the tube, which would otherwise arise from the heaving of the vessel.

Public attention has frequently been called to the invaluable use of the barometer in indicating a coming storm. A notable instance of the advantage arising at sea from attending to its indications, is given by Dr Arnott in his admirable treatise on Physics, where he says: The mercury in the tube had been observed by the captain to be falling with great rapidity, on which he gave orders to prepare with the utmost haste for an approaching storm. The extent of the preparations, and the promptness with which they were ordered to be executed, astonished the crew, the most skilful of whom had not as yet perceived in the sky or clouds the most distant threatening of a storm. But before the preparations were completed, though executed with the utmost expedition, one of the most tremendous storms ever experienced by the oldest seaman on board had burst on the noble vessel; the noise of the thunder, the wind, and the waves, was such that no voice could be heard on board, and even the trumpet sounded in vain. When the storm had passed, the sea was strewed with wrecks; but our noble ship, though sadly buffeted by the storm, still survived. Had it not been for the timeous warning given by that small tube of mercury, neither the strength of the ship nor the skill of her commander would have been sufficient to have saved one man alive to tell the sad tale.

The barometer is, then, an instrument of the highest importance to the mariner; and its indications, combined with the appearance of the clouds, direction of the wind, and height of the thermometer, ought to be made by him a study of first importance, in order that he may be able to take full advantage of its timeous warnings, and thereby avoid many disastrous consequences of the fierce storms which he frequently encounters at sea. But to do so successfully, it is necessary, in the first place, that he have an instrument whose indications may be depended upon. The instruments generally used are those

constructed by Messrs Negretti and Zambra, instrumentmakers to Her Majesty, the Royal Observatory, the Admiralty, and the British Meteorological Society, 1 Hatton Garden, London.

In order to render their barometers permanently useful, and as little as possible liable to derangement, they have introduced the following changes in the regular instruments, which may fairly come under the head of important improvements. In the first place, those intended for public stations and general use are mounted in a frame of solid oak, firmly screwed together; and, in all, the brass or ivory scales that barometers are generally furnished with, are here replaced by a substantial plate of porcelain, on which the degrees and figures are legibly engraved and permanently blackened in, so that, as far as the divisions and figures are concerned, there will be no danger of their becoming faded or obliterated. This is a very important improvement, especially for an instrument that has to be placed in an exposed position, where the mariner may be able to consult it at any time, even in the middle of the night, should he require to do so.

The mercurial tube of the barometer is of large diameter, so as to render the mercury easily visible, and shew the slightest variation, and is so constructed, that the liability of air entering it, is, we may say, entirely obviated. Should air by any means find its way up the tube of the barometer to the vacuum at the top, let the quantity be ever so minute, the indications of the instrument will be erroneous, and can no longer be relied on. To prevent the admission of air to the vacuum above the mercury, a trap is laid at the bottom of the tube, near the part called the cistern, so that if any air should find its way into the tube, it cannot possibly pass the trap, but is there detained, and the instrument is in no way deteriorated or injured by its presence. These tubes, after being filled, are, moreover, boiled. The operation of boiling a barometer tube consists in placing the tube, after it is filled with mercury, over a charcoal fire, and retaining it there till the mercury boils. It is an opera