128 



making a special appeal to theory, for Davy was expressly sent in 

 quest of the means, and, after much thinking and many careful 

 experiments upon the gas and upon flame, he found it in the shape 

 of these two items of knowledge : No mixture of gases will take 

 fire below a certain temperature ; and wire gauze conducts the 

 heat away so rapidly as to keep it below this temperature. As 

 long as the knowledge and the arrangements necessary to render 

 it useful as a means to an end existed in Davy's mind, it was theory 

 to himself; after his lamp was made and tested on a small scale, 

 there were doubts for some time whether such a simple contrivance 

 would be sufficient to meet the want generally ; but now the theory 

 is converted into a rule for practice. Again, in Newcomen's steam 

 engine, at every stroke the steam was condensed by a jet of cold 

 water ; but this cooled the cylinder too, and of every five parts of 

 steam which afterwards entered, nearly four were employed in 

 raising the cylinder to the proper heat, and only one part did 

 effective work in moving the piston. To prevent this waste of 

 steam — that is waste of fuel — was a very desirable end, and Watt 

 found the means one Sunday when he was taking a walk, in the 

 shape of this item of knowledge — steam is an elastic fluid, which 

 means in simple language, that if that room were air-tight, and full 

 of steam, and if this room were air-tight, and contained nothing, 

 except, perhaps, luminiferous ether, and if the door were opened, 

 the steam in that room would stretch itself out until it filled both 

 rooms. Not many minutes afterwards everything was sketched out 

 in Watt's mind. He had a separate condenser connected with the 

 cylinder; he saw the cylinder filled with steam, the condenser 

 exhausted of its air, connection opened between the two, the steam 

 rushing into the condenser to shrink into a drop of water, and the 

 water which accumulated removed by the same pump as the air. 

 When Mr. Kendall decided that the relation between theory and 

 practice was that of thinking and doing, he must have had some 

 example like this in his mind. It would be impossible to find in 

 the history of mankind an instance where thinking, pure and 

 simple, has resulted in so much doing. Well, Monday morning 

 came, and Watt hastened to test his theory by experiment ; before 



