magnetic observations made in the Antarctic expeditions of Sir J. C. Ross, and Lieutenants Moore and Clerk, published in the Philosophical Transactions, in Nos. V., VI., and VIII. of the Memoirs entitled "Contributions to Terrestrial Magnetism."

The practical application of these formula to the correction of the deviations of the standard compass, is divested of all technical difficulty, and is rendered extremely simple and easy, by the forms and tables published by the Admiralty under the title "Instructions for the Computation of a Table of the Deviations of a Ship's Compass, from Deviations observed on 4, 8, 16, and 32 points, and for the Adjustment of the Table on a Change of Magnetic Latitude," by Archibald Smith, Esq.: London, 1850. It is therefore unnecessary to enter upon it here, further than to say that the corrections are made by the aid of five co-efficients, A, B, C, D, E, of which B and D are also required for the correction of the dip and force.

As the "Instructions" do not give the formula for correcting the dip and force, it may be convenient to reprint them here.

In these formulæ, symbols are used which have the following significations is the total magnetic force of the earth observed on shore; the inclination observed on shore by Fox's apparatus; the true magnetic azimuth of the ship's head counted from the magnetic North, positive when her head is West of North, and negative when East; p', e', and ', are the same elements observed by the needles of the standard compass and of Fox's apparatus on board; c, d, and A', are co-efficients to be employed in calculating the corrections applicable to the magnetic observations of the declination, inclination, and force made at sea in any part of the globe.

7. For the correction of the inclinations observed at sea the coefficients c and d may be computed from the disturbances on the several points, shown by the observations of the inclination with Fox's apparatus when the ship was swung in harbour, by the formula:

c cos+d tan

=

(1D) sin cosec 'tan

for all other points than North and South, and with the ship's head North or South, by

c cos + d tan 6

=

(cos +B) sec S'tan ';

observing that the values of employed should be calculated by the method given in Mr. Smith's "Instructions." With the values of c and d thus obtained, tables for the correction of the inclinations observed on different courses at sea in all values of may be computed by the formula :

observing that the first must be used when the ship's course is between N.E. and S.E., or N.W. and S.W.: and the second when her course is nearer the N. and S. points; and that the values of 'should be calculated by means of the declination-coefficients.

8. The coefficient A', for the correction of the intensities of the force observed at sea, may be computed by the subjoined formulæ, from the observations of the intensity with Fox's apparatus on board with the ship's head on the different points, compared with those of the same instrument on shore, and with the absolute value of the total force on shore (ø) obtained in the manner described in App. 3. Tables for the correction of the intensities observed at sea may also be constructed by the same formulæ :

the first to be used when the inclination is large, and the second when the inclination is small.

9. If the disturbances of the compass-needle, at the spot where the gimball-table for Fox's apparatus is fixed, differ materially from those at the spot where the standard compass is fixed, the standard compass may be removed temporarily to the gimball-table, and the deviations observed on the eight principal points, or on the N.W., S.W., S.E., and N.E. points, which will give the values of B and D for that spot.

SECTION III.

HYDROGRAPHY.

BY CAPTAIN F. W. BEECHEY, R.N.

Making a Passage.

THE observer's attention is directed first to those objects which affect the passage of a vessel from one part of the globe to another; such as the movement, the duration, the limits, and the periodic occurrences of those great currents of the atmosphere and of the ocean, upon which the speedy and successful issue of a passage mainly depends.

Well recorded and established facts bearing upon the several points connected with these inquiries are highly important to navigation, and may be collected by every assiduous seaman in the ordinary course of his duties.

1. It is well known that in various parts of the globe there exist monsoons, and zones of trade and variable winds; and that these and other disturbances of the atmosphere which influence the surface of the ocean are the principal causes of the many currents which sweep over the face of the earth. The effect of these upon a vessel passing to and fro is one of the most

useful inquiries a seaman can make; and as both (wind and current) perform an important part in the

economy of nature, an additional interest attaches to a correct knowledge of them. The seaman should therefore not only carefully note the direction and force of the winds, but should connect with such entries notices as to when and where any continued or periodic wind commenced and terminated; what was its strength and effect upon the passage; whether it came on suddenly, and was furious while it lasted, or otherwise; whether it was preceded by any particular symptoms, and whether it was such as usually occurs at that season; and lastly, whether it be advisable to cross this wind in any particular direction, such as close hauled or large, &c.

2. To detect the current, a more than ordinary attention must be paid to the reckoning of the ship: the compass by which the course is steered should occasionally be compared with that by which the variation is determined, in every position of the ship's head ;* and the ship's place should be determined by observation at least once a day. Sights for chronometer morning and evening should both be referred to noon, at which time the latitude will of course be observed; and all observations for latitude at night, or for fixing the ship's place at any time, should be referred to one period of the day, in order that the position of the ship by observation, as compared with her place by the Dead Reckoning, may give the direction and force of the current, if any, for the twenty-four hours. These observations should all be entered in a table, and at the close of certain obvious and natural periods of a passage, such as that of entering or emerging from the trade-wind, the calm latitudes, the com

* See Section 'Terrestrial Magnetism.'

mencement or termination of the monsoon, of any positive change of current, or from any continued state of things to another, the whole effect of the current for the period should be deduced, and an average of its daily rate and set be given, together with any remarks which may be considered useful.

3. With the direction of the current thus determined, it is very desirable to connect the temperature of the surface of the sea, for it has been by such observations that we have been able to trace, with a certainty amounting almost to proof, the continuous course of the same body of water for thousands of miles over the troubled surface of the ocean, and that other curious and important facts in physical hydrography have been ascertained. We would therefore urge attention to the subject as one of considerable importance to navigation. As a proof of its influence upon a passage, we need only instance the remarkable phenomenon of the Equatorial and Guinea currents : two streams in contact, but flowing in opposite directions, and having a temperature differing 10 or 12 degrees from each other, and yet pursuing their opposite courses for upwards of a thousand miles; and according as a vessel is placed in one or the other of these currents, will her progress be aided or retarded from 40 to 50 miles a day.*

Could we but obtain a register of the temperature of the surface of the sea from every ship in active service, we should be able in a short time to construct tables showing the normal temperature of the surface of the ocean for every 5° of latitude for every month in the year, and a comparison of these with the actual * Sabine's 'Hydrographical Notices.'