web.  The clinkers and ashes left after the combustion of the coal, are precipitated into the ash-pit, where the chain turns down over the roller at the extremity of the furnace.  In Messrs. Maudslays’ plan of a self-feeding furnace the fire bars are formed of round tubes, and are placed transversely across the furnace.  The ends of the bars gear into endless screws running the whole length of the furnace, whereby motion is given to the bars, and the coal is thus carried gradually forward.  It is very doubtful whether any of these contrivances satisfy all the conditions required in a plan for feeding furnaces of the ordinary form by self-acting means, but the problem of providing a suitable contrivance, does not seem difficult of accomplishment, and will no doubt be effected under adequate temptation.

164. Q.—­Have not many plans been already contrived which consume the smoke of furnaces very effectually?

A.—­Yes, many plans; and besides those already mentioned there are Hall’s, Coupland’s, Godson’s, Robinson’s, Stevens’s, Hazeldine’s, Indie’s, Bristow and Attwood’s, and a great number of others.  One plan, which promises well, consists in making the flame descend through the fire bars, and the fire bars are formed of tubes set on an incline and filled with water, which water will circulate with a rapidity proportionate to the intensity of the heat.  After all, however, the best remedy for smoke appears to consist in removing from it those portions which form the smoke before the coal is brought into use.  Many valuable products may be got from the coal by subjecting it to this treatment; and the residuum will be more valuable than before for the production of steam.

STEAM.

165. Q.—­Have experiments been made to determine the elasticity of steam at different temperatures?

A.—­Yes; very careful experiments.  The following rule expresses the results obtained by Mr. Southern:—­To the given temperature in degrees of Fahrenheit add 51.3 degrees; from the logarithm of the sum, subtract the logarithm of 135.767, which is 2.1327940; multiply the remainder by 5.13, and to the natural number answering to the sum, add the constant fraction .1, which will give the elastic force in inches of mercury.  If the elastic force be known, and it is wanted to determine the corresponding temperature, the rule must be modified thus:—­From the elastic force, in inches of mercury, subtract the decimal .1, divide the logarithm of the remainder by 5.13, and to the quotient add the logarithm 2.1327940; find the natural number answering to the sum, and subtract therefrom the constant 51.3; the remainder will be the temperature sought.  The French Academy, and the Franklin Institute, have repeated Mr. Southern’s experiments on a larger scale; the results obtained by them are not widely different, and are perhaps nearer the truth, but Mr. Southern’s results are generally adopted by engineers, as sufficiently accurate for practical purposes.