Mr. Henwood on the Cornish Pumping-Engines. 491 



Many subjects which are yet undetermined have pressed on 

 my attention during these experiments, among which the 

 steam-case and air-pump are not the least important. 



If any condensation take place in the case, when protected 

 from the influence of the external air, it must be by radiation 

 to the rarer steam within the cylinder. Now such influence, 

 if exerted during at least two-thirds of every stroke*, would not 

 only not increase the force of the engine by adding to the elas- 

 ticity of the steam, but would render requisite the injection of a 

 larger quantity of cold water into the condenser to effect con- 

 densation, and thereby add to the burden of the air-pump f. 



There must be a point at which the resistance ot vapour, 

 not abstracted, to the descent of the piston, and the pressure 

 of the atmosphere on the air-pump whilst discharging its load, 

 are at a minimum. Beyond this, if it be attempted to reduce 

 the force of the vapour, by injecting more cold water, the 

 burden of the air-pump is increased by the exposure of its 

 piston to the atmosphere for a longer time during its discharge ; 

 whilst on the other hand, if it be sought to lessen the duration 

 of atmospheric pressure on the air-pump, by diminishing the 

 quantity of cold water introduced into the condenser, the in- 

 creased elasticity of the unabstracted vapour offers a greater 

 resistance to the descent of the piston J. 



This subject presents many inviting topics of inquiry; but 

 the pursuit of them, and the earlier preparation of the details § 

 which I have now the honour to submit to the Institution, 

 have been prevented by more pressing occupations. 



4, Clarence Street, Penzance, W. J. HENWOOD. 



August 30th, 1837. 



* See Table IV, 



+ Brewster's Edinburgh Journal of Science, O. S. IX., p. 162. 

 + Ibid., X. p. 40. 



^ A short notice of these experiments appeared in Brewster's Edinburgh 

 Journal of Science, N. S., VI. p. 246. 



