LOESELIA, in botany, from Joseph Loesel, a genus of the Didynamia An giospermia class and order. Natural or der of Convolvuli, Jussieu. Essential character ; calyx four-cleft ; corolla with all the segments directed one way ; sta mina opposite to the petal; capsule three celled. There is but one species, viz. L. ciliata, found at La Vera Cruz in South America.
LOG, in naval affairs, a machine used to measure the rate of a ship's velocity through the water. For this purpose, there are several various inventions, but the one most generally used is the fol lowing, 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 per pendicular 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 quarter minute, is to an hour of time), is wound about a 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 mi nute-glass by another, the mate of the watch fixes the pin, and throws the log over the stern, which, swimming perpen dicularly, 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 mea sured from a mark on the line, at the dis tance of twelve or fifteen 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 then being on the log dislodges the pin, so that the board now presenting only its edge to the water is easily drawn aboard. The numberof 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 de'_ermine any variation in either of them, and to make allowance according ly. if the glass runs thirty seconds, the distance between the knots should be fifty feet. When it runs more or less, it should, therefore, be corrected by the following analogy. As thirty is to fifty,
so is the number of seconds of the glass to the distance between the knots upon the line. As the heat or moisture of the weather has often a considerable effect on the glass, so as to make it run slower or faster, it should be frequently tried by the vibrations of a pendulum. As many accidents attend a ship during a day's sailing, such as the variableness of wind, the different quantity of sail carried, &c. it will he necessary to heave the log at every alteration; but if none of these al terations be perceptible, yet it ought to be constyntly heaved. In ships of war and East lndiamen, it is usaal to heave the log once every hour, and in all other vessels once in two hours; and if at any time of the watch the wind has increased or abated in the intervals, so as to affect the ship's velocity, the officer generally makes a suitable allowance for it at the close of the watch.
LOG board, a table generally divided into five columns, in the first of which is en tered the hour of the day; in the second, the course steered ; in the third, the num ber of knots run off the reel each time of heaving the log; in the fourth, from what point the wind blows ; and in the fifth, observations on the weather, variation of the compass, &c.
Lou book a book ruled in columns like the log-board, into which the account on the log-board is transcribed every day at noon ; from whence, after it is corrected, &c. it is entered into the journal.
Los wood, in the arts, is derived from a low prickly tree, which is found in great plenty at Campeachy, in the bay of Hon duras, and is denominated " liwmatoxy lon campechianum." It comes to Europe in large logs, cleared from the bark, and is very hard, compact, heavy, and of a red colour. It is in high request among dyers, especially in dyeing black. It gives out the colour both to water and alcohol; the liquor at first assumes a fine red co lour with a shade of purple. The infu sion becomes gradually deeper, and at last almost black. To cloth previously boiled in alum and tartar, it gives a beau tiful violet colour, which, however, will not stand. Alkalies render the colour darker, acids change it to yellow. From a variety of experiments it is found, that the colouring matter of log-wood bears in many respects a strong analogy to tan nin, but in others it differs from it.