THE subject of the Mariner's Compass has for many years past been considered a most important one, and has had great attention bestowed on it, especially since the introduction of iron as a material for building ships. Large masses of iron, of necessity, cause local disturbance to the indications of ships' compasses, and, without doubt, many vessels and a vast number of human lives have been lost through such disturbance.

The subject is so important a one, owing to the magnitude of the interests affected by it, that we hope, in an early Number or Numbers, to give an Article on it by one of those scientific men whose attention has been especially devoted to it.

The principles of construction of compasses for boats are the same as of those for ships ; but, on account of the more violent motion of a boat in a rough sea, it is necessary to provide some mode of steadying the face-card to which the needle is attached, in addition to the ordinary "gimbals" on which ships' compasses are balanced. For such compasses in boats and small vessels, subjected to the motion of a rough sea, are so perpetually in motion, spinning round and round, as to be then altogether useless. It is evident, therefore, that they are not appropriate to life-boats, whose work always lies amongst heavy seas.

To remedy this evil, the late Mr. F. DENT, the eminent chronometer-maker, of 61 Strand, London, devised the particular description of compass known as " Dent's Fluid Compass," the improvement consisting in the card and needle being immersed in a fluid enclosed in a hermetically-closed bowl. This plan was found to answer the purpose admirably, and these compasses are now in general use in the Royal Navy, and to some extent in the mercantile marine; but their expensive character has prevented their being universally so, and they were also of an inconveniently large size for the use of boats, more especially in times of emergency, such as the desertion of a leaky, or stranded, or burning ship, when all the space available within a boat is often required for the stowage of passengers and provisions.

Our object in this Paper is not to enter scientifically into any part of the subject of either ships' or boats' compasses, but to leave the same for a future occasion: we only allude to it to introduce to the notice of the maritime public an admirable modification of DENT'S Fluid compass and binnacle especially designed for and under the superintendence of the ROYAL NATIONAL LIFE-BOAT INSTITUTION.

This compass and binnacle is shown in the accompanying illustration, which will speak for itself almost without description.

DENT'S FLUID LIFE-BOAT COMPASS.

The only modification in the compass is the reduction of the size of the card, which is only three inches in diameter. The binnacle therefore so portable as to be readily held in the hand. What more need we state in its favour in order to convince every is made of copper, japanned; it is scarcely i one of its manifest advantages, than, that its more than 6 inches square, and 8 inches ; cost is only 41. 4s., and that it has been high, yet works on double gimbals; it is i found, in the life-boats of the NATIONAL only 51 Ibs. in weight, yet is provided with ! LIFE-BOAT INSTITUTION, to answer admirably in every respect?