Besides this valve, another of the very opposite kind is sometimes used. Upon stopping the engine, and extinguishing the fire, it is found that the steam condensed within the boiler produces a vacuum; so that the atmosphere, pressing on the external surface of the boiler, has a tendency to crush it. To prevent this, a safety valve is provided, which opens inwards, and which being forced open by the atmospheric pressure when a vacuum is produced within, the air rushes in, and a balance is obtained between the pressures within and without. (71.) We have already explained the manner in which the governor regulates the supply of steam from the boiler to the cylinder, proportioning the quantity to the work to be done, and thereby sustaining a uniform motion. Since, then, the _consumption_ of steam in the engine is subject to variation, owing to the various quantities of work it may have to perform, it is evident that the PRODUCTION of steam in the boiler should be subject to a proportional variation. For, otherwise, one of two effects would ensue: the boiler would either fail to supply the engine with steam, or steam would accumulate in the boiler, from being produced in too great abundance, and would escape at the safety valve, and thus be wasted. In order to vary the production of steam in proportion to the demands of the engine, it is necessary to increase or mitigate the furnace, as production is to be augmented or diminished. To effect this by any attention on the part of the engine-man would be impossible; but a most ingenious method has been contrived, of making the boiler regulate itself in these respects. Let T (fig. 40.) be a tube inserted in the top of the boiler, and descending nearly to the bottom. The pressure of the steam on the surface of the water in the boiler forces water up in the tube T, until the difference of the levels is equal to the difference between the pressure of the steam in the boiler and that of the atmosphere. A weight F, half immersed in the water in the tube, is suspended by a chain which passes over the wheels P P´, and is balanced by a metal plate D, in the same manner as the float in fig. 32. is balanced by the weight A. The plate D passes through the mouth of the flue E, as it issues finally from the boiler; so that when the plate D falls, it stops the flue and thereby suspends the draught of air through the furnace, mitigates the fire, and diminishes the production of steam. If, on the contrary, the plate D be drawn up, the draught is increased, the fire rendered more effective, and the production of steam in the boiler stimulated. Now suppose that the boiler is producing steam faster than the engine consumes it, either because the load on the engine has been diminished, and therefore its consumption of steam proportionally diminished, or because the fire has become too intense. The consequence is, that the steam beginning to accumulate, will press upon the surface of the water in the boiler