TACHOMETER. This is the name of an apparatus patented by Messrs. Napier, in 1818, for ascertaining the speed with which vessels pass through the water. It consists of a limi zontal spindle, moving freely on pivots attached to the side of the vessel beneath the water line, and enclosed in a case open at both ends. The spindle is fitted with vanes, the pitch or angle of which is so regulated, that ten revo lutions of the spindle shall equal one fathom. Other wheel-work connects this spindle with a dial and index, so as to measure the number of nautical miles which the apparatus, and con sequently the ship itself, has passed through the water.
Messrs. Penn, the engineers of Greenwich, patented in the same year a tachometer of a very ingenious and peculiar kind. It is to be attached to an auxiliary steam ship ; that is, a ship which sails only under ordinary circum stances, but which has also facilities for steaming in ordinary weather. (Such an aux
iliary moving power is described under SCREW PROPELLER.) Messrs. Penn's apparatus is intended, not only to measure the speed of such a vessel through the water, but to strengthen or weaken the steam force ac cording as the sailing force is weak or strong.
It is self-acting, like the governor and valve of a steam engine : it feeds itself with steam power, according to its wants. There is a small horizontal paddle-wheel attached to the side of the ship, and the shaft of this wheel is connected, by intervening apparatus, with the steam pipe ; as the vessel advances this little wheel rotates; and according as the rotation is fast or sloW (depending on the greater or less speed), the steam pipe becomes opened less or more widely.