IN our last issue we gave a brief description of the ordinary ship's log, -which has been in use during the last 275 years, and of patent logs also in use for the last half century. In continuation of that article, we have now to describe the latest instrument placed in the hands of the navigator, to automatically inform him, at any and every moment, the distance his ship has run from time to time—the electric log.

That wonderful, subtle, immaterial essence, termed electricity, which permeates all matter, and which science is year by year loosening from its hidden bonds, and subjecting and guiding to the use of man, has now been enlisted in the seaman's aid,-and whilst it is carrying our very words and thoughts from continent to continent along the ocean's bed, is, or shortly will be, counting and recording the miles of those viewless tracks which unnumbered craft are cleaving through the surface of the azure main.

An electric log was invented and patented in 1876, by Mr. C. E. KELWAY, of Portsea. Others had been previously designed and patented, but failed on practical trial, and never came into use.

Mr. KELWAY'S log was similar in form and general character to the patent logs of Massey and Walker, and was towed, pre- cisely as they are, from the stern of a ship. Its chief feature was the making and breaking an electric circuit by means of a screw, or spindle, with spiral blades, revolving in the water in the same manner as Massey'e and Walker's logs, and an electric battery connected with the stop- motion indicator. This log has been tried in several of Her Majesty's ships, but, we are informed, with unsatisfactory results, it being found difficult to prevent the ingress of water to within the cylinder, especially on a ship's way being stopped, when the log, sinking to the extent of the line, would necessarily be subjected to proportionately increased pressure. A train of wheels in the cylinder, connected with indicators or hands, showed the tenths, units, and hundreds of miles, precisely as in the patent logs above referred to.

We understand that Mr. KELWAY has now introduced a modification of his elec- tric log, in which the electric apparatus is placed inboard, in a shaft or tube pass- ing through the ship's bottom or side, whence the bladed screw is projected at will into the outside water, and rotates in proportion to the velocity with which the vessel is passing through it, as in the towing logs. The electric apparatus is, we believe, above the surface of the water, which latter, of course, rises in the open tube to the sea-level outside the ship.

Whether this arrangement will be open to the same objections that have been made to BERTHON'S log, referred to in our last number, experience alone can deter- mine.

We now come, however, to the latest electric log, which it is our object to more particularly describe. This log was In- vented and patented by Mr. E. M. LOWNE, scientific instrument maker, and is brought oat by Messrs. Dollond, of St. Paul's Churchyard, the old-established and well- known manufacturers of optical, astro- nomical, and scientific instruments.

By this invention, the distance a ship travels through the water is continuously ,and, accurately registered, by means of dials sad indicators, in any part of the vessel, or in any number of places at the same time from the same log. The log and apparatus are of a very durable and extremely simple character, not liable to get out of order, and easily managed.

The rotating log in the sea telegraphs to the indicators, and registers on board every tenth of ft mile travelled, about one hundred and eighty revolutions of the log being equal to the tenth of a mile The telegraphing mechanism of the log is hermetically closed -within the log itself, so that sea-water cannot possibly gain admittance.

The Jinn, to which the log is attached, does not rotate, but contains the necessary wires, properly insulated, for conducting the current of electricity to the indicators on board.

The log may remain overboard during an entire voyage, the hauling in for ad- justment and register being unnecessary, thereby effecting a great saving of labour.

Should the" log become foaled, it would be immediately detected by means of the indicators (one of which should be placed close to the ship's compass, where it would be constantly under the eye of the man at the wheel), thereby avoiding un- discovered and accumulative errors pos- sible with all other mechanical logs, which may become fouled and remain so. a con- siderable time before being discovered.

This log is so constructed that the weight is much greater at the end attached to the line, while the end furthest from the line is an air-chamber: the great advan- tage thus gained is, that the log will not tow out of the water even with only ten fathoms of line, bat is inclined to dive, the more so the quicker it is drawn through the water. la the neck of the log are placed friction-wheels, which reduce the friction to a minimum, and ensure uniformity of motion; whilst the neck of the log, to- gether with the friction-wheels, are covered •with a tube of brass, and this again is strongly attached to the line by a ball- and-socket joint.

The electricity is generated by a one- cell Leolanche battery, and so small is the quantity of electricity expended at each contact, that it would take years to exhaust the same, so that there need be no fear of the apparatus requiring frequent attention.

Both the tow-line wires and the wires used for conducting the electric current inboard are thoroughly insulated, after which the covering is subjected to a pre- paration of ozokerite, which has been proved to resist the action of sea-water.

The log will also be invaluable in ascer- taining the rate of currents in channels, &o,, under all conditions of weather, the results being telegraphed on shore and recorded by the indicators in any con- venient place.

Another use to which tMs invention can be applied is the registration of wind currents, or, in other words,, it could be applied to the anemometer.

Mg. 1 represents the log attached to the tow-line. It is important to notice f that the cylinder in front of the fens does l

not rotate; this motion commences at the break immediately in front of the fans; the front cylinder is merely a covering for the friction-wheels and insulated wire, as well as forming a good finish to the log itself, Fis. 2.

Fig. 2 represents the face of an indicator; the outer circle is divided into j tenths of a mile, and the whole circle makes up a total of ten miles, the two smaller dials taking up the enumeration, one to 100 miles, and the other to 10,000 miles.

The cost of DOLLAND'S log is15l 15s., including the tow-line, battery, recording indicator, &e., necessary for fitting up a vessel. That of Mr. KELWAY is, we under- stand, very much more, besides, in his last design, having the disadvantage, of requiring an open tube, several inches in diameter, passing through the ship's bottom or side, below the surface of the water. DOLLAND'S patent electric log has been tested oa several voyages with, we are informed, very satisfactory results.

Whether, or to what extent, it will prove to possess advantages over KELWAY'S log, time only can show.