The following are convenient Forms for Registering the Observations for Determining the Deviation of the Standard Compass.

Table for converting Points of the Compass, and their Fractional Parts, into Degrees, etc.

To Convert a True Course into a Compass Course.

Easterly Variation is
Westerly Variation is
Easterly Variation is
Westerly Variation is

from all points between N. and E....S. and W. to all points between N. and E......S. and W. to all points between N. and W......S. and E. from all points between N. and W...S. and E.

To Correct the Course at once for Variation
and Deviation.

If they are both of the same name, i.e. both E. or both W., add one to the other, and apply the sum according to their joint name. Example:-Compass course N.N.E.; given, the deviation 8° 10' E., and the variation 20° 10' E., the sum thereof 28° 20′ E., being applied to the right hand of N.N.E., gives the true course, N. 50° 50' E. But if one be E. and the other W., take their difference, give it the name of the greater, and apply it according to that Example:-Suppose the deviation be still 8° 10' E., but with the variation of 20° 10′ W., their difference 12° 0' West, applied to the left hand, will give N. 10° 30′ E. for the true course.

name.

SIR WILLIAM THOMSON'S COMPASS.

THE following description of this Compass, which is now largely in use in the Mercantile Marine, and is also supplied to H.M. ironclad ships, is taken from a paper read by the inventor at the United Service Institution.

To produce such steadiness of the compass-card as has hitherto been obtained in steamers which have powerful engines, and where there is much vibration, it has been customary to suspend the bowl by means of india-rubber bands. A serious objection to this method is that the india-rubber is liable to become rotten by exposure to heat or oil, especially if it is used in fine enough bands to give the requisite steadiness in all circumstances. After many trials of metallic springs in lieu of the india-rubber, I at last found a plan of brass spring resembling a rope grummet (A, Figs. 1, 2, 3, 4, 5, 6), but with elastic brass wire instead of the rope strands, by which I succeeded in obtaining more satisfactory steadiness of the compass than with india-rubber. The construction of this brass grummet-ring and the mounting of the compass bowl upon it, may be described as follows:-A single wire is first bent and its ends are united by soldering or brazing, so as to form a ring of the proper size. This serves as a core on which a second brass wire is laid spirally, six turns round the core (Fig. 6). The ends of this second wire are also united by soldering or brazing, and thus an elastic ring is produced strong enough to support the compass bowl. The compass bowl is suspended from the elastic ring with the intervention of a rigid gimbal ring. The elastic ring has two sockets fixed at the ends of a diameter, which rest on two balls attached to the brass rim of the binnacle stand (Figs. 3 and 4). The elasticity of the ring mitigates the effect on the knife edges bearing the gimbal ring and bowl, and on the point bearing the compass card, of vertical tremors of the platform on which the binnacle rests. The knife edges of the gimbal ring are supported on two grooved stirrups, hung by chains from the elastic rings (Figs. 5 and 6). This suspension mitigates the effect of horizontal tremors of the platform.

Figs. 1, 2, 3, 4, 5, 6 illustrate the suspension of the bowl from the elastic ring.

For ascertaining the heeling error I use an auxiliary instrument for comparing the vertical component of the earth's magnetic force on shore with the vertical component on board ship. This instrument is constructed as follows:-Two magnetic needles of hardened steel wire are joined together and supported on two iridium points in a line at right angles to the lengths of