818 A method of cooling the Air of [No. 10, new series. 



But however specious this application may appear, it fails alto- 

 gether in practice, for several reasons ; first, the difference between 

 the perfect expansive action of the air undergoing compression in 

 the pump-barrel, and the imperfect expansive action of the same 

 air in the expanding working-barrel. In order to insure any effect 

 at all, a considerable degree of compression must be employed, or 

 the force of the expanding air will not be equal to the friction of 

 the additional piston, and other apparatus required for its introduc- 

 tion. The action of compressed air would be very similar to that 

 of high pressure steam, and that not being a profitable source of 

 power, until the tension has reached several atmospheres, the air 

 should not be employed at less. But then the difficulty is expe- 

 rienced, that the air having been forced into the cooler by the pump , 

 and deprived then of its heat of compression, occupies less space 

 than before; and this deficiency increases with the compression? 

 as does also deficiency from leakage. While too the air, in under- 

 going compression, was offering resistance to the power from the 

 very commencement of the stroke, and that power was not produc- 

 ing any effect that could be considered useful, until when, towards 

 the end of the stroke, the compression in the cylinder, exceeding 

 that in the cooler, the air begins to be forced therein ; it is to be 

 remembered that owing to the imperfection of the best cylinders, 

 valves, and such apparatus that can be made, only a small propor- 

 tion of the expansive force of the air or steam can be employed, 

 and the part so lost, increases with the compression adopted. 



Again ; from the excessively varying resistance of air when un- 

 dergoing great compression, such violence is done to all parts of 

 the pump, that it cannot continue long to work tight and true — 

 and this was the principal reason of the abandonment of the Croy- 

 don atmospheric line ; the air, in undergoing compression, came 

 so suddenly to a maximum of resistance, as to resemble an explo- 

 sion in its effect. The great inertia and small elasticity of water, 

 render that fluid most appropriate for being raised by pumps act- 

 ed on by natural powers ; but the small inertia and great elasti- 

 city of air, render it almost impossible to be dealt with continually 

 at high degrees of tension. 



But the most important objection to the employment of a high 

 degree of compression, which is necessary to the introduction of 



