16 MR. WATT'S INVENTIONS FOR THE IMPROVEMENT 



meter and six inches stroke or range of motion of the piston) 

 exposed a greater surface to condense the steam, than the cy- 

 linders of larger engines did, in proportion to their respective 

 contents. By shortening the column of water in the pump 

 worked by the engine, and by thus diminishing its weight, 

 lessening the amount of work to be performed, less steam and 

 less injection-water were required, and the model worked at 

 a proper speed. 



The purpose for which the model was put into Mr. Watt's 

 hands was thus accomplished ; and with this result an ordi- 

 nary workman would have been satisfied. Mr. Watt, how- 

 ever, having now become aware of the great consumption of 

 steam by the engine, his inquiring mind was impelled to 

 make a more accurate investigation of the causes of this con- 

 sumption, in which he proceeded in a manner truly philoso- 

 phical. He ascertained, from experiments made with boilers 

 of various constructions, that the quantity of steam obtained 

 in a given time from boiling water, is neither in proportion 

 to the surface of the water, nor to the quantity of the water, as 

 had been supposed, but that it is in exact proportion to the 

 quantity of heat which enters into the water, and depends 

 on that alone ; and he discovered, further, that the heat 

 which enters the water depends chiefly on the quantity of sur- 

 face exposed to the action of the fire ; or, in other words, to 

 the size of the bottom of the boiler and of the other parts of 

 it on which the fire plays. He likewise determined the weight 

 of coal required to be burned in order to evaporate any given 

 quantity of water. Being convinced that great errors existed 

 in the statements that had been previously made of the increase 

 of bulk in water when converted into steam, he proceeded to 

 examine this point by experiment; and he discovered that water, 

 converted into steam having the same heat as boiling water, was 

 expanded to 1 800 times its former bulk ; or, as a rule for ready 

 approximate calculation, that a cubic inch of water produced a 

 cubic foot of steam. He constructed a boiler to be applied to 

 his own model, which would show, by inspection, by merely 

 looking at it, the quantity of water evaporated, and consequently 

 would enable him to calculate the quantity of steam used, in every 

 stroke of the engine. This quantity he now proved to be seve- 

 ral times the cylinder-full. He also observed that whenever 

 an attempt was made to improve the vacuum in the cylinder, 

 by throwing in more injection-water, the power gained did 

 not increase in proportion to the expenditure of steam. It 

 occurred to him that the cause of this was the boiling of 

 water, in a vacuum, at very low heats, (then recently deter- 

 mined by Dr. Cullen, to be under 100 degrees of Fahrenheit's 



