(151.) Besides presenting to the action of the fire a sufficient surface to produce steam at the required rate, the capacity of the boiler must be proportioned to the quantity of water to be evaporated. The space within the boiler is appropriated to a twofold purpose: 1st, To contain the water to be evaporated; 2dly, To contain a quantity of ready made steam for the supply of the cylinder. If the space appropriated to the steam did not bear a considerable proportion to the magnitude of the cylinder, the momentary expansion of the steam passing to the cylinder from the boiler at each stroke would reduce the pressure of the steam in a great proportion, and unless the pressure in the boiler were considerably greater than that which the steam is intended to have in the cylinder, the pressure in the latter would be reduced below the proper amount. The proportion of the [Pg264] steam space in the boiler to the magnitude of the cylinder has been very variously estimated, nor can it be said that any practical rule of a general kind has been adopted. It is held by some that the steam-space will be sufficient if it contain five times the quantity of steam consumed at each stroke, while others maintain that it should contain at least ten times that quantity, and opinions vary between these limits. (152.) The proportion of water-space in the boiler to its evaporating power should also be regulated, so that the introduction of the feed at a comparatively low temperature may not unduly chill the water in the boiler. Supposing the feed to be introduced in a low pressure boiler at the temperature of 100°, and that the necessary temperature within the boiler be 225°, the quantity of water it contains should be about five times the quantity evaporated, and therefore also five times the quantity introduced through the feed per hour. For every cubic foot of water per hour therefore, intended to be evaporated by the boiler, water-space for five cubic feet should be provided. It is, however, right to repeat that this (like almost every other so called rule) is the result not of any exact general calculation, but one deduced from the custom which has obtained among the manufacturers of steam-engines.