ITS our last January Number we gave a short account and illustration of the Boat's Fluid Compass, adopted by this Institution, and in this Number we insert an interesting paper on the " Mariner's Compass," with which we have been favoured by Captain EVANS, Superintendent of the Compass Department of the Admiralty. We now add the following diagram and explanation which will make sufficiently clear to the general reader the character of a fluid compass as manufactured by Mr. F. DENT, of the Strand, London:— Fig. 1.

Fig. 2.

Figures 1 and 2 are vertical sections; Fig. 1 of the Fountain Reservoir, and Fig. 2 of the Bowl when filled. The same letters refer to similar parts in each figure. A is the bowl filled with dilute alcohol, or other suitable liquid; B is the card with the magnetic needle attached to it, and turning on a central point OT pivot -, c is the glass top of the bowl; D is the annular fountain reservoir. This reservoir has a nozzle or filling hole, closed by a screw R, below which at £ is a hole, forming a communication with the bowl A.

To fill the bowl, the screw H is taken out, and a small funnel inserted, through which the liquid is poured in (the air passing out at the same time by the side of the funnel), until the bowl A is full.

The liquid is still poured in until it nearly fills the reservoir D ; the screw K is then screwed in, and the whole is thus tightly closed up. The elasticity of the air remaining in the reservoir allows for the contraction and expansion of the liquid; and if any liquid escapes by evaporation from the screws not being absolutely air-tight, its place is supplied by liquid from the fountain reservoir D.

If by any chance a bubble of air should find admission to the bowl A, it passes up into the reservoir D, either spontaneously or by slightly inclining the bowl to allow of its escape, which it does instantly.

The fluid is composed of one-third spirits of wine and two-thirds pure water. In high latitudes more spirits may be used if there is danger of its freezing..