Times Of Observation

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; but from a variety of

stances--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 phænomena 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 phænomena 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 most rapid variation. _But should any one of those 21st days

fall on a Sunday, then it will be understood that the observations are

to be deferred till the next day, the 22nd._ The series of observations

on board each vessel should commence at 6 o'clock A.M. of the appointed

days, and terminate at 6 A.M. of the days following, according to the

usual reckoning of time adopted in the daily observations.

In addition to the twenty-five hourly readings at the solstices and

equinoxes as above recommended, it would be desirable to continue the

observations until a complete elevation and depression of the barometer

had been observed at these seasons. This plan is adopted at the Royal

Observatory, Greenwich, and would be attended with this advantage were

it generally so--the progress of the elevation and depression would be

more readily traced and their velocities more accurately determined than

from the four or eight daily readings.