AMONGST the many appliances which con- tribute to the safe navigation of a ship, none is of more importance than the small instrument termed " a log," by which the distance run can be approximately ascer- tained from day to day, or at any moment.

When the sun, moon and stars are visible, the position of a ship on the earth's surface can be learned by astronomical observation, and indeed more accurately than by means of the log and compass, termed " dead reckoning," since a ship's course may have been diverted by leeway, or obstructed or accelerated by ocean currents, neither the force nor direction of which may have been correctly calculated.

Nevertheless the sky may be overcast and neither sun, moon nor stars seen for many days, as was experienced by St. Paul and his shipmates before they were wrecked on the island of Malta; and on such occa- sions the log alone must be relied on to ascertain the distance run.

Every one who has made a sea voyage will be familiar with the character of a log; but, for the information of those who have not had that advantage, it may be briefly described as a small flat piece of thin wood, in shape the quadrant of a circle, with a radius of about six inches, and having a strip of lead fixed round its arc j or curved side, so as to float in a perpen- , dicular position. It is attached to a small! line, termed a log-line, which is wound on i a reel, the latter being held by a seaman I over his head whilst an officer or seaman throws the log and line over the stern on the windward side.

Within about three feet of the log, or log-ship, as it is called, the log-line branches into two, one end of which passes through a hole at one end of the i curved side of the log-ship, and the other end is attached to a peg of hard wood or | bone, which is firmly pushed into another | hole at the other end of the curved side.

i The log-line is marked with knots forty - | eight feet apart, which distance bears ! nearly the same proportion to a mile that twenty-eight seconds does to an hour.

When, therefore, the log is thrown over- board, as soon as a certain length of the line termed stray line has run out, indi- cated by a piece of rag or bunting, and the latter passes through the hand of the I thrower, a sand-glass of twenty-eight i seconds is instantly turned, and on the last ! sand running out, the line is held fast.

. The pressure of the water on the log-ship I then causes the withdrawal of the peg, , and the former falls flat on the water's | surface, so that it can be readily hauled on board, and the length of line that has run out, shown by the number of knots which have passed through the hand of the thrower, indicates the number of miles per hour at the rate of which the ship is running at the moment.

The knots are not made in the log- line itself, but in a short piece of small cord spliced into it at the proper distances apart, each successively with an additional knot in it, and intermediately between each two of these knotted cords is one with a single knot, representing half knots or miles. Thus, if when stopped the knotted cord with five knots and the intermediate single knot had passed through the hand, the same would indicate that the vessel was running at the rate of five and a half knots or sea miles an hour. .

When going at high rates of speed, a sand-glass of fourteen seconds is used, and each knot of the line then represents two miles, and each half knot one mile.

Simple and primitive as this contrivance appears to be, it does not seem to have been invented until the early part of the seventeenth century, previous to which time navigators, when beyond the sight of land, were solely dependent on their compasses and on astronomical observa- tions to ascertain the position of their ships on the earth's surface, merely guess- ing the distance they had run from day to day, and being even guided, as is recorded of Columbus, in 1492, by the flight of birds as an indication of the direction of the land.

In 1578 Bourne, in a work entitled " Inventions and Devices," describes in his Twenty-first Device a plan for ascertaining the distances run by ships, which was an approach to Massey's Log, invented two hundred and fifty-six years later. It does not, however, appear to have come into use.

Furchas states that the ordinary log above described, and still universally adopted, was first introduced in 1607, but did not come into general use until many years later. The inventor was a benefactor of mankind, but his name is unknown.

We now come to the era of patent logs.

The first of these was patented in the year 1834 by Mr. MASSEY, and it is now in very general use in our merchant service, and to a great extent in the Royal Navy, but we believe a modification of it, patented at a later period by Mr. WALKER, is in more general use in the latter service. Each may be briefly described as a brass cylinder from 14 to 18 inches long and 2 to 2J inches in diameter, containing within it clockwork machinery consisting of a train of wheels, and having three dials with hands or in- dicators similar to the dial and hands of a watch or clock. They are towed by a strong line, about fifty fathoms long, from the stern of a ship, and just as the hands of a watch show the seconds, minutes and hours of time as they pass, so these dials notify tenths, units, and hundreds of miles.

Continuing the comparison, as an ordi- nary household clock is kept in motion by a heavy weight, and a watch by the force exerted by a delicate spring, so the wheel-work of the patent log is set and kept in motion by the passage of the ship dragging the log with it through the water. This is effected by the pressure of the water on three blades or fans, like those of the propellers of screw steamers, which are attached, diagonally, to the outer revolving case or cylinder, which sets the interior wheel-work in motion, the rapidity of that motion being of course proportional to the revolutions of the outer case, to which the blades are attached, which, again, is proportional to the pressure of the water on the blades, being greater or less in accordance with the greater or less speed of the ship.

Logs of this description must of course be hauled on board the ship, in order to ascertain, by inspection of the dials and indexes, the distance ran since the last previous inspection. They are also liable, when in tow, to be fouled by any spars, and their gear or other floating bodies lying in the track of a ship, and have not unfrequently even been seized and damaged or destroyed by sharks folloiriflg one. These uncertainties attending this and all other patent towing logs up to the present time have prevented their superseding the old original log, which is still used in all ships as a check on the patent ones.

To obviate the above imperfections, a very ingenious pressure-log was invented and patented in 1849 by the Rev. E. L. BERTHON, the present Vicar of Romsey, the well-known inventor of the collapsible life-boat with which our naval troopships and other vessels, both British and foreign, are provided, and who is also the author of several other useful inventions.

We are not provided with the details of Mr. BERTHON'S log or with the necessary diagrams to illustrate it, but it may be briefly defined as a small tube passed through the keel or bottom of a ship, projecting some eight inches below it, and having an open hole in it facing the direction of the motion of the ship. The pressure on the water in the tube through this hole is conveyed upwards inboard, and ultimately connected with a mercurial column and the speed accurately shown at any moment by the rise and fall of the mercury, read off on a vernier as in a barometer. A few of these instruments are fitted to vessels in the Royal Navy, but they have not been brought into general use.

As in towing-logs there is a risk of injury from contact with other bodies, so in the case of one immediately in contact with the hull of a ship there is the draw- back that when proceeding at high speed the disturbance of the water rushing in, vertically as well as laterally, to occupy the space vacated by the advancing hull, must more or less cause a general dis- turbance of the water only too likely to affect any pressure-gauge in actual con- tact with the latter.

Another step in advance has now been made by the introduction of electric-logs, with the object of enabling the indications of even a towing-log to be read off from indexed dials placed in any part or. dif- ferent parts of a ship without hauling in the log. Thus the captain of a ship might at any moment, without leaving his cabin, or even rising from his seat, ascertain to a tenth of a mile the distance his ship had run.

The electric-log is, however, so com- plete an innovation on all previous ones, and is so deeply interesting, that we pro- pose to devote a separate article to it in our next Number.