172 OF THE POWER OF STEAM, &c. [SECT. iv. 



cylinder ; or a loss equivalent to 13 horse power in an air pump of the size in 

 the example. To reduce this loss one-half by the mode proposed in art. 352 

 is certainly worthy of attention. 



355. It is important to remark the circumstances which contribute to this 

 loss of power. The loss is proportional to the capacity of the pump ; therefore the 

 smaller it is the better, provided it be sufficient to take the air. The friction is 

 four-sevenths of the power, the actual resistance of the vapour nearly three- 

 sevenths, and that of the water about one thirty-second. The resistance is greater 

 the smaller the passages and valves are, but such increase does not affect the 

 whole power in a material degree. The increase of the size of the air pump, 

 beyond the proportions I have given, can give advantage only in an ill-constructed 

 and leaky engine ; but its decrease, after a very short range, reduces the power 

 considerably. 



