CHAP, v.] THE STEAM-ENGINE. 401 



heat in spite of the absence of smoke. In one word, a furnace called 

 smoke-consuming is not necessarily the most economical. 



To return to the boiler. We have seen what is the form of the 

 principal body and the two heaters. The latter are filled entirely 

 with water, which reaches to a certain height in the boiler. The free 

 space which is above the level of the water is filled with the steam 

 before it passes to exert its force on the machinery of the engine : it 

 is called for this reason the reservoir or steam-space. 



The steam-space ought to bear a certain proportion to the capa- 

 city of the boiler, which is found to be in practice about one-third. 

 The reason for the large size of the reservoir arises from the necessity 

 of drying the steam formed as much as possible, for it almost always 

 .entangles minute particles of liquid which ought not to be introduced 

 into the cylinder. With regard to the proper size of the whole boiler, 

 that should be made in proportion to the quantity of steam to be 

 generated in an hour under ordinary working conditions. The 

 force which steam at a high temperature possesses, and which 

 is exerted first of all on the inside of the boiler, requires in this 

 a power of resistance which cannot be obtained without certain 

 conditions as to form, thickness, and quality of materials used. 

 One of the best forms, as regards resistance, is the cylindrical, 

 terminated at both ends by hemispheres, The material generally 

 adopted is wrought-irori of the best quality, most carefully joined 

 with rivets of great solidity. It appears that steel is beginning to 

 be substituted for iron, but only in certain parts of the boiler : but 

 this is chiefly a question of cost. 



Some years ago in France there was an official rule to regulate 

 the thickness of the wrought-iron plates according to the mean 

 pressures, calculated in atmospheres, that each boiler was called upon 

 to bear, which is interesting as showing the experience of the best 

 French engineers. The rule in question was this : Add to 3 milli- 

 metres the product of T8 millimetres by the greatest working pressure 

 expressed in atmospheres and the diameter of the boiler in metres. 1 



1 Applying this rule to a boiler of l'20 m diameter, destined to support a pressure 

 of 4 atmospheres, and the whole thickness would be 3 mm -f r8 mm X 1'20 X 4'5 = 

 127"" n . 



D D 



