SCIENTIFIC AND TECHNICAL LABOURS TO 1850. 117 



career produced the sparks. This method of measuring 

 velocity by the help of marks, which an electric spark 

 brands on polished steel or sprinkles on sooty steel 

 surfaces, has maintained its ground, and is still to-day 

 employed especially for measuring the velocity of pro- 

 jectiles in large and small gun-barrels. 



The suggestion of storing up the unemployed 

 heat of one operation for use in the succeeding 

 operation, derived from my brother William's descrip- 

 tion of the Stirling hot-air engine, which I received 

 in the year 1845. interested me in a very high 

 degree. It appeared to me to open the way into a 

 yet unknown vast domain of technical science. It 

 occurred at a time when the idea, pervading and 

 governing the physical science of this age , of the 

 causal connection of all natural forces unconsciously 

 swayed men's minds, until it soon after became through 

 Mayer and Helmholtz common scientific property. The 

 principle of the circulation of heat in working engines 

 arid of the heat - equivalent of work already found 

 clear expression in my paper "On the application of 

 heated air as motive power", whose publication was 

 occasioned by Stirling's engine. I consider the chief 

 value of this essay however to have been, that 

 it incited my brothers William and Frederick to 

 their later pioneer efforts in the province of thermal 

 economy. 



In my first- dial telegraph of 1846 I consequen- 

 tially carried out the principle of the automatic inter- 

 ruption of the electric current both for the apparatus 



