AND APPLICATION OF THE STEAM-ENGINE. 17 



thermometer) ; and consequently, that at greater heats, the 

 cold injection-water itself was made to boil, and converted 

 into steam, in the cylinder, and resisted the descent of the 

 piston. The great consumption of steam by the engine, Mr. 

 Watt now clearly perceived, proceeded from its being chilled 

 and condensed by the coldness of the cylinder, (and that in 

 a greater degree in proporton to the quantity of injection- 

 water employed), before that vessel had become sufficiently 

 heated to retain it in an elastic state to prevent its con- 

 densation into water : this determination led immediately to 

 the practical conclusions, that, in order to derive the greatest 

 advantage from this agent, the cylinder should always be pre- 

 served as hot as the steam that entered it; and that, when the 

 steam was condensed, it should be cooled down to 100 degrees 

 of Fahrenheit, or lower, in order to render the vacuum com- 

 plete and to preserve it in that state. Early in the year 1765, 

 the fortunate thought occurred to him, in consequence of the 

 steady and continued application of his mind to the subject, of 

 accomplishing this object, by condensing the steam in a separate 

 vessel, exhausted of air, and kept cool by the injection of cold 

 water; between which and the cylinder a communication was 

 to be opened, while the cylinder itself was to be kept con- 

 stantly hot, so that the steam which entered it would always 

 remain steam, while it continued in the cylinder. No sooner 

 had this design occurred to him, than the means of effecting it 

 presented themselves to his mind in rapid succession ; and thus 

 was achieved the invention which was the main foundation of 

 the fame Mr. Watt subsequently acquired ; and which has 

 since been applied, with so much labour and ingenuity, to the 

 advancement of human affairs*. 



* The above review of the successive steps by which Mr. Watt proceeded 

 to his great improvement upon the principle of the Steam-engine, is derived, 

 with a few omissions and verbal alterations, from the details on that subject 

 given in the memoir of Mr. Watt, published in Professor Napier's Supple- 

 ment to the Encyclopaedia Britannica ; and which, that gentleman in- 

 forms us, in an editor's note to the article, he received " from a quarter 

 which entitles him to state, with the utmost confidence, that it contains an 

 accurate and faithful account of Mr. Watt." As first delivered at Hazehvood 

 it formed part of the concluding lecture of a brief course on the Steam- 

 engine, addressed, as mentioned above, to my elder pupils in that establish- 

 ment. They had been prepared for these details, by some familiar experi- 

 ments on the production and condensation of vapours ; the boiling of liquids 

 at low temperatures when relieved from the pressure of the atmosphere, as 

 evidenced by a modification of the instrument called the Pulse-glass ; the 

 explanation and use of Dr. Wollaston's instrument showing the fundamen- 

 tal principle of the Steam-engine ; a sectional drawing of Newcomen's en- 

 gine, and other appropriate means. The progress and final result of Mr. 

 Watt's investigations was illustrated by sectional drawings: and by a moving 

 sectiontil diagram of his double-acting engine, which had been constructed 

 by Mr. Edwin Hill, now of Bruce Castle, in the year 1821. 



C 



