A patent or taffrail log is very different in principle as well as in construction. It records the actual distance passed over by the ship and not the speed at any particular time. It con sists of a register having three dials or other in dicating devices, and appropriate gearing, a rota tor which has a small body and blades like a propeller—but of greater proportional surface— and a log-line of hard braided untarred hemp connecting them. As the ship goes ahead, the rotator—which is a hundred yards astern—tends to twist the log-line; this in turn transmits the twisting motion to the register which. by suitable gearing, records the proper distance. In early types of talTrail logs there was a tendency of the log-line to twist until the inertia of the gearing was overcome and then the latter would buzz around for two or three seconds. In recent types this is avoided by a little fly-wheel on the shaft to which the line is attached; this makes the motion smooth and regular and adds to the ac curacy of the instrument. The three dials usual ly fitted en the register record tenths of miles, miles. and tens of thus if the dials show 6-1 S. the nading of the register is 61.8 miles. Ily reading the log at intervals the speed may be ascertained: the reading is usually recorded every hour in the ship's log-book (q.v.).
The log and line for determining the speed of ships is of comparatively reeent origin. Purchas refers to it in 1607, but it 'ruts certainly not common before 1620. No record is to he found in any ancient writin,gs of the meatus used by old navigators to estimate the speed of their ships.
Bourne, writing in 1577, describes a design for a machine to record the speed of ships not unlike :Nlassey's submerged log, which later had the rotator or propeller directly connected to the dial and the whole submerged and secured to a. braes or trailing lines; it had to be hauled on board to read it, and this was a fatal defect, which soon paused it to be displaced by the taff rail it was never much itscul. Taff rail logs came into general use about 1875. On large steamers the rev.dutimis of the propeller give a fairly au-curate measurement of the dis tance passed over and of the speed; and if the weather is fair and sea not too rough the meas urement is as accurate as that of any form of log. provided the corresponding speeds for various conditions el dr:night. trim, etc.. have Tile difficulty of preparing tables in which the requisite information is t'inhodIcil is such Ihat the II )1_! is almost always used, the revolutions of the engine being reusaaled as a check on it in case it gets out of order or he eomes fouled with seaweed.
in shallow waters when the currents are swift and variable a grounil-loq is used. In this case a light lend or other weight is hitehed to the log-line instead of the chip. This sinks to the bottom and the speed of the vessel over the groundl is ascertained instead of her speed through the watc.r, and some idea of the direction she is making over the ground is also thus ob tained.