116 THE DEVELOPMENT OF THE MODERN STEAM-ENGINE.
altitude being determined by the relation existing between
the force of gravity and centrifugal force in the temporary
position of equilibrium. The distance from the point of sus-
pension down to the level of the balls is always equal to 9.78.
inches divided by the square of the number of revolutions
per second-i.e., h =9.78 1/N2
The arms carrying the balls, or the balls themselves, are pinned to rods, MM', which are connected to a piece, NN', sliding loosely on the spindle. A score, T, cut in this piece engages a lever, V, and, as the balls rise and fall, a rod, W, is moved, closing and opening the throttle-valve, and thus adjusting the supply of steam in such a way as to preserve a nearly fixed speed of engine. The connection with the throttle-valve and with the cut-off valve-gear is seen not only in the engraving of the double-acting Watt engine, but also in those of the Greene and the Corliss engines. This contrivance had previously been used in regulating water- wheels and windmills. Watt's invention consisted in its application to the regulation of the steam-engine.
Still another useful invention of Watt's was his " mer- cury steam-gauge"-a barometer in which the height of the mercury was determined by the pressure of the steam in- stead of that of the atmosphere. This simple instrument consisted merely of a bent tube containing a portion of mercury. One leg, BD, of this U-tube was connected with the steam-pipe, or with the boiler by a small steam-pipe; the other end, C, was open to the atmosphere. The pressure of the steam on the mercury in BD caused it to rise in the other “leg" to a height exactly proportioned to the press- ure, and causing very nearly two inches difference of level to the pound, or one inch to the pound actual rise in the outer leg. The rude sketch from Farey, here given (Fig. 30), indicates sufficiently well the form of this gauge. It is still considered by engineers the most reliable of all forms of steam-gauge. Unfortunately, it is not conveniently ap-
