_Answer._ By an instrument called a _steam-gauge_. There are a great variety of such instruments made, but they may all be divided into two classes, and they all operate upon one of two principles. In the one class the pressure of the steam acts upon a diaphragm or plate of some kind, as shown in fig. 77, which represents a section of a gauge of this kind; _a b_ is a metal plate made with circular corrugations, as shown in section and also by the shading in fig. 78, which represents a front view of the gauge with a part of the dial-plate removed. The steam enters by the pipe _c_ and the small opening _d_, and fills the chamber _e_ behind the metal plate or diaphragm. The corrugations of the latter give it sufficient elasticity, so that when the pressure is exerted behind it, it will be pressed outward by the steam. If it were flat, it is plain that it would not yield, or only to a very slight degree, to the pressure of the steam. In the centre of the diaphragm on the outside is a pin or stud, _f_, which bears against the plate. This stud is attached to a bent lever or “_bell-crank_,”[39] _g h k_, whose fulcrum is at _h_. To the outer end, _k_, a rod, _l_, figs. 77 and 78, is attached, the lower end of which is connected to the short arm _m_ of a toothed segment, _n_, whose fulcrum is at _o_. This segment gears into a small pinion, _p_, which is attached to a spindle or shaft, which carries a pointer, fig. 78. It is obvious now that if the diaphragm, _a b_, is pressed outward, it will move the bent lever _k h g_, the motion of which will be communicated by the rod _l_ to the toothed segment _n_, which will in turn revolve the pinion _p_, and thus move the hand or index. We have selected for this illustration one of many forms of this kind of gauge. The mechanical appliances for communicating the motion of the diaphragm to the index or pointer are different in the gauges made by different manufacturers. The form of the diaphragm also differs. In some cases it is made of a metal plate; in others a spiral spring is used, covered with india-rubber to make it steam-tight. The steam-gauge represented by figs. 77 and 78 is the form manufactured by M. B. Edson, of New York. [39] A _bell-crank_ is a lever with an elbow in it. [Illustration: Fig. 79.] [Illustration: Fig. 80.]