A machine or apparatus used to measure the rate of a ship's velocity through the water. For this purpose there are various inventions; but the one mostly used is the following, and called the common log. It is a piece of thin board, forming the quadrant of a circle of about six inches radius, and balanced by a small plate of lead nailed on the circular part, so as to swim perpendicularly in the water, with the greater part immersed. The log line is fastened to the log by means of two legs, one of which is knotted through a hole at one corner, while the other is attached to a pin fixed in a hole at the other corner, so as to draw out occasionally. The log line, being divided into certain spaces, which are in proportion to an equal number of geographical miles, as a half or a quarter minute is to an hour of time, is wound upon the reel. The whole is employed to measure the ship's head-way in the following manner; the reel being held by one man, and the half minute glass by another, the mate of the watch fixes the pin, and throws the log over the stern, which swimming perpendicularly, feels an immediate resistance, and is considered as fixed; the line being slackened over the stern to prevent the pin coming out.
The knots are measured from a mark on the line, at the distance of 12 or 15 fathoms from the log; the glass is therefore turned at the instant that the mark passes over the stern; and as soon as the sand in the glass has run out, the line is stopped; the water being then on the log, dislodges the pin, so that the board now only presenting its edge to the water, is easily drawn aboard. The number of knots and fathoms which had run off at the expiration of the glass, determines the ship's velocity. The half-minute glass and divisions on the line should be frequently measured, to determine any variation in either of them, and make an allowance accordingly. If the glass runs 30 seconds, the distance between the knots should be 50 feet When it runs more or less, it should, therefore, be corrected by the following analogy; - as 30 is to 50 so is the number of seconds of the glass to the distance between the knots upon the line. The heat or moisture of the weather having often a considerable effect upon the glass, so as to make the sand run faster or slower, it should be frequently tried by the vibration of a pendulum.
The inventor of this simple and admirable contrivance is unknown; and no mention of it occurs till the year 1607, in an account of an East India voyage, published by Samuel Purchas. Since that period, the log has been in general use, and many improvements have been made upon it. One of the most conspicuous of these improvements, is that invented by Mr. James Hookey, a midshipman in the navy, who received a honorary medal from the Society of Arts for the same.
The advantages gained by Mr. Hookey's invention are, that it gives the distance the ship runs more correctly, as it remains more stationary in the water than the one generally in use; and when required to be hauled into the ship, by giving it a sudden jerk, the toggle swivels round, and disengages the line from the spring, in consequence of which, the log ship reverses its position, and may then be pulled into the ship with the greatest ease. With respect to the lines, Mr. Hookey recommends, that they be saturated in a composition of oil, which makes them more buoyant and pliant, and prevents kinking; it likewise prevents their contracting, which in a new line is about 20 feet in 50 fathoms. As many serious accidents are likely to occur by getting a false depth of water, in consequence of the contraction of the line attached to the lead, it becomes an object worthy of attention to prevent the possibility of such accidents taking place. The log is formed like a fish. Fig. 1 represents one running out, and Fig. 2 the same, in the act of being pulled in; r the toggle, s the spring; the eye of the line is put on the toggle, which is then pushed under the spring; the flap board t falls down, and the fish runs out.
When the line is taught, a sudden jerk will make the toggle pass the spring and let go the line; the fish then swings round, the flap board t closes, and it is easily pulled in. Fig. 3 shows the under side; the flap-board t is jointed to the fish by the strap of the copper v, which passes round a pin llll, and this pin is held by the copper strap w; the line is attached to the log by a loop which goes in at the mouth, and is held by a peg which forms the eye; the flap-board t, if made of copper, has a piece of wood rivetted to it in the middle to stiffen it; if made of wood, a slip of lead or copper x is rivetted on, to make it heavy enough to drop down readily when thrown into the water. Fig. 4 is a top view of one made thin and wide, like a flat dish; the spring s, which holds the toggle is underneath, beneath the fish and the flap-board t; the spring may be above or below in either case. The following are the instructions given for using the log-ship. The eye in the line is to be put over the toggle, on the tail of the fish, and when the line is all run out from the reel, and it becomes taught, by giving it a sudden jerk, the toggle will swivel out; the fish will then reverse its position, float on the surface of the water, and may be hauled into the ship with the greatest ease.
When it is necessary to shift the line at the head of the fish, knock out the peg that forms the eye, and the line will then disengage itself; and in attaching another line, make an eye in it, and pass it into the mouth of the fish perpendicularly, through which put the peg that forms the eye, and it will be quite secure. The inventor strongly recommends that all log lines, and lines to the lead, should be saturated for one hour in linseed and lamp 'oil, three-fourths of the former, and one-fourth of the latter well mixed together, after which, hang them up to dry; contraction will thus be prevented, and they will be pliable and buoyant.