The spring _d_ must therefore pull down on the end of the lever with a tension equal to 70 pounds. When the pressure of the spring bears directly on the valve, as shown in fig. 76, then the tension of the spring must be just equal to the pressure on the valve. This tension is produced by screwing down the nuts, _c_, _c_. The spring _d_, which produces the requisite pressure on the end of the safety-valve lever, fig. 74, is arranged inside of two cylinders, _g_ and _h_, which slide over or into each other like the sections of a telescope. This arrangement is called a _spring-balance_. The spring, _d_, is attached to the covered ends and draws them towards each other. The upper cylinder _g_ is connected by a rod, _i_, to the flattened end of the lever _c′_, which has a hole drilled through it to receive the rod. The other end of the rod is screwed into the upper cylinder _g_. This rod is sometimes arranged so that it can be either lengthened or shortened by the nut _j_. By lengthening or shortening the distance, the tension of the spring is either diminished or increased. The lower cylinder of the spring-balance, represented in fig. 74, is attached to a lever, _m_, which is fastened to the back of the _steam-gauge k_. This is shown more clearly by fig. 75, which represents the back of the gauge, and also the lever, _l m_, whose fulcrum is at _m_. The spring-balance is attached to the lever at _k_. By drawing down the lever the tension of the spring is increased, and by raising it up it is diminished. The lever is held in any desired position by the latch, _n_, and the ratchet _r r_. By this contrivance, which is employed on the engines built at the Grant and also at the Baldwin Locomotive Works, the pressure on the valve can at once be either increased or diminished, which it is often desirable to do, especially when an engine is not at work. The spring-balance is shown in fig. 71, and is indicated by the letter _M_ and the lever by _N_. Unless the pressure of the steam exceeds that on top of the valve, it will of course not be opened. As there is always danger that a safety-valve or some of its attachments may become corroded or otherwise disordered, so that it will not act promptly or with certainty, it is desirable to open it frequently, so as to be sure that it is in good working order. To do this the pressure on the valve must be reduced below that in the boiler, which can very conveniently be done with the _spring-balance_ lever which has been described.