The Hurricane Guide | Page 8

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; but from a variety of circumstances--the nature of the service in which the vessels may be employed, particular states of the weather, &c.--such a course of unremitting labour cannot be expected; it is therefore necessary to fix on some stated hours at which the instruments before particularized should be regularly observed throughout the voyage, and their indications faithfully recorded. The hours of 3 A.M., 9 A.M., 3 P.M., and 9 P.M., are now so generally known as meteorological hours, that nothing should justify a departure from them; and it is the more essential that these hours should be adopted in the present inquiry, because the series of observations made at intervals terminated by these hours can the more readily be used in connexion with those made contemporaneously on land, and will also serve to carry on investigations previously instituted, and which have received considerable illustration by means of observations at the regular meteorological hours; we therefore recommend their general adoption in all observations conducted at sea.
It is intended in the sequel to call attention to particular parts of the earth's surface where it is desirable that additional observations should be made, in order to furnish data of a more accurate character, and to mark more distinctly barometric changes than the four daily readings are capable of effecting. The best means of accomplishing this for the object in view appears to be the division of the interval of six hours into two equal portions, and to make the necessary observations eight times in the course of twenty-four hours. In the particular localities to which allusion has been made we recommend the following as the hours of observation:--
A.M. 3, 6, 9, noon. P.M. 3, 6, 9, midnight.
In other localities besides those hereafter to be mentioned, when opportunities serve, readings at these hours would greatly enhance the value of the four daily readings.
There are, however, portions of the surface of our planet, and probably also ph?nomena that occur in its atmosphere, which require still closer attention than the eight daily readings. One such portion would appear to exist off the western coast of Africa, and we recommend the adoption of hourly readings while sailing to the westward of this junction of aqueous and terrestrial surface; more attention will be directed to this point as we proceed. There are also ph?nomena the localities of which may be undetermined, and the times of their occurrence unknown, but so important a relation do they bear to the subject of our inquiries, that they demand the closest attention. They will be more particularly described under the head of accumulations of pressure preceding and succeeding storms, and minute directions given for the hourly observations of the necessary instruments. In the mean time we may here remark that hourly observations under the circumstances above alluded to are the more important when we consider that the barometer, the instrument employed in observing these moving atmospheric masses, is itself in motion. The ship may meet the accumulation of pressure and sail through it transversely; or she may sail along it, the course of the vessel being parallel to the line marking the highest pressure, the ridge or crest of the wave; or the ship may make any angle with this line: but whatever the circumstances may be under which she passes through or along with such an accumulation of pressure, it should ever be borne in mind that her position on the earth's surface is scarcely ever the same at any one observation as it was at the preceding, the barometer in the interval has changed its position as well as the line of maximum pressure, the rate of progress of which it is desirable to observe. It will, therefore, be at once apparent that in order to obtain the most accurate data on this head hourly observations are indispensable. To these readings should of course be appended the places of the ship from hour to hour, especially if she alter her course much.
There is another point to which we wish to call attention in immediate connexion with hourly readings--it is the observation of the instruments on the days fixed for that purpose: they were originally suggested by Sir John Herschel, whose directions should be strictly attended to: they are as follows:--
The days fixed upon for these observations are the 21st of March, the 21st of June, the 21st of September, and the 21st of December, being those, or immediately adjoining to those of the equinoxes and solstices, in which the solar influence is either stationary or in a state of