William Radcliff Birt

“ Sir John Herschel has suggested that "the bulb of the thermometer should be so situated as to afford the best chance of its indicating the exact mean of the whole barometric column, that is to say, fifteen inches above the cistern enclosed within the case of the barometer, nearly in contact with its tube, and with a stem so long as to be read off at the upper level."
Previous to making an observation with the barometer the instrument should be slightly tapped to free the mercury from any adhesion to the glass; any violent oscillation should, however, be carefully avoided. ”

“ Redfield has shown that on some occasions storms have been preceded by an unusual pressure of the atmosphere; the barometer has stood remarkably high, and it has hence been inferred that there has existed around the gale an accumulation of air forming a margin; barometers placed under this margin indicating a much greater pressure than the mean of the respective localities. With regard to the West Indian and American hurricanes—any considerable increase of pressure, especially within the space marked out to the eastward of the 50th meridian, will demand immediate attention. ”

“ Now the connexion of the barometer with this phase of the storm is falling with the wind from left to right, the observer facing the centre while the first half is transiting. [1] During the latter half these conditions are reversed, the observer still keeping his position, his face directed to the S.W., the barometer rises with a N.W. wind, which rushes past him from his right to his left hand with a decreasing force. We have therefore a rising barometer with the wind from right to left during the latter half of the storm, the observer having his back to the centre. ”

“ Should any advantage be found to result from the study of the Atmospheric Waves, as explained and recommended in this little work, or the seaman be induced by its perusal to attend more closely to the observations of those instruments that are calculated to warn him of his danger, an object will be attained strikingly illustrative of the Baconian aphorism, "Knowledge is Power." ”

“ Without going over the ground so well occupied by those able writers on the subject of storms—Redfield, Reid, Piddington, and Thom—it will be quite sufficient for our present purpose simply to notice the essential phænomena of revolving storms as manifested by the barometer and vane. ”

“ The barometer on shipboard should be suspended on a gimbal frame, which ought not to swing too freely, but rather so as to deaden oscillations by some degree of friction. To the upper portion of the tube in this construction of instrument light is alike accessible either in front or behind, and the vernier is furnished with a back and front edge, both being in precisely the same plane, nearly embracing the tube, and sliding up and down it by the motion of rack-work; by [Pg 20] the graduation of the scale and vernier the altitude of the mercury can be read off to ·002 inch. ”

“ The usual indications of a storm in connexion with these instruments are the falling of the barometer and the freshening of the wind, and it is generally considered that a rapid fall of the mercury in the hurricane regions invariably precedes the setting in of a storm.
There are three classes of phænomena that present themselves to an observer, according as he is situated on the line or axis of translation, or in either the right or left hand semicircle of the storm. These will be rendered very apparent by a little attention to the annexed engraving, fig. 1. ”

“ It has been observed in the south-east of England that the barometer has generally passed a maximum on or about the 3rd of every month, and this has been so frequently the case as to form the rule rather than the exception. The same fact during a more limited period has been observed at Toronto. With especial reference to this subject the three-hourly series of observations may be resorted to in all localities, but especially north of the 40th parallel in the northern hemisphere. ”

“ Vessels navigating the Archipelago [Pg 41] between China and New Zealand, should make observations every three hours, in order that the undulations arising from the configuration of the terrestrial and oceanic surfaces may be more distinctly marked and more advantageously compared with the Gulf of Mexico, the Caribbean Sea, and the northern portion of the African continent.
The Pacific Ocean.—As this ocean presents so vast an aqueous surface, generally speaking observations at intervals of six hours will be amply sufficient to ascertain its leading barometric phænomena. ”

“ Under any circumstances, and in any locality, a high barometer not less than a low one should demand particular attention, and if possible, hourly readings taken some time before and after the passage of the maximum: this will be referred to more particularly under the next head.
Preceding and Succeeding Accumulations of Pressure.—Mr. Redfield has shown in his Memoir of the Cuba Hurricane of October, 1844, that two associated storms were [Pg 47] immediately preceded by a barometric wave, or accumulation of pressure, the barometer rising above the usual or annual mean. ”

“ The letters outside the thick circle are intended to distinguish the points of the compass, and in use should always coincide with those points on the chart. The letters within the thick circle indicate the direction of the wind in a hurricane, the whirl being shown by the arrows between the letters. In the northern hemisphere the direction of the whirl is always contrary to that in which the hands of a watch move, and in the southern coincident thereto. The graduation is intended to assist the mariner in ascertaining the bearing of the centre of a storm from his ship. ”

“ The vernier should then be adjusted to the upper surface of the mercury in the tube; for this purpose its back and front edges should be made to coincide, that is, the eye should be placed in exactly the same plane which passes through the edges; they should then be brought carefully down until they form a tangent with the curve produced [Pg 21] by the convex surface of the mercury and the light is just excluded from between them and the point of contact. It is desirable in making this adjustment that the eye should be assisted by a magnifying-glass. ”

“ When they are laid flatly upon a chart, the continuity of the lines on the chart is not materially interfered with, while the idea of a body of air rotating in the direction indicated by the arrows is conspicuously presented to the mind. ”

“ There can be no question that the greatest amount of information, the accuracy of the data supplied, and in fact every meteorological element necessary to increase our knowledge of atmospheric waves, may be best obtained by an uninterrupted series of hourly observations made on board vessels from their leaving England until their safe arrival again at the close of their respective voyages ”

“ Scotland may contribute most important information on this head by a regular and, as far as circumstances will allow, an uninterrupted series either of six-hourly or three-hourly observations. The intervals of observation on board vessels stationed at the Western Isles, the Orkneys, and the Shetland Isles, ought not to be longer than three hours, principally on account of the great extent of oscillation observed in those localities. Vessels arriving from all parts of the world as they approach the United Kingdom should observe at shorter intervals than six hours. ”

“ Vessels staying at Cape Verd Islands should not omit to make observations at three hours' interval during the whole of their stay, and when circumstances will allow, hourly readings. At the Canaries, Madeiras, and the Azores, similar observations should be made. Vessels touching at Cape Cantin, Tangier, Gibraltar, Cadiz, Lisbon, Oporto, Corunna, and Brest, should also make these observations while they are in the localities of these ports. At the Scilly Isles we have six-hourly observations, made under the superintendence of the Honourable the Corporation of the Trinity House. ”

“ Assuming that the hurricane sets in at the ship's place with the wind at S.E., the proceeding will be altogether different. At first the wind is fair for the prosecution of the voyage, and it is desirable to take advantage of this fair wind to avoid as much as possible the track of the centre, which passes over the ship's place in this instance, and is always the most dangerous part of the storm. ”

“ As such comparisons will always be made when the vessel is in port, sufficient time can be allowed for making the requisite number of observations: hourly readings would perhaps be best, and they would have the advantage of forming part of the system when in operation, and might be accordingly used as such.
It is not only desirable that the zero point of the barometer should be well determined in the first instance; it should also be carefully verified on every opportunity which presents itself ”

“ In the previous sketch of the localities for the more important observations, it will be seen that within the [Pg 42] tropics there are three which demand the greatest regard.
I. The Archipelago between the two Americas, more particularly comprised within the 40th and 120th meridians west longitude, and the equator and the 40th degree of north latitude. As a general principle we should say that vessels within this area should observe the barometer every three hours. ”

“ They exercise an important influence on temperature. It has also been shown that the neighbourhood of water has a very considerable influence in increasing the oscillations of the mercurial column in the barometer, and in the great systems of European undulations it is well known that these oscillations increase especially towards the north-west. ”