636 TRANSACTIONS OP THE AMERICAN INSTITOTE, 



scends the compass of any language it would be within my power 

 to employ. 



But, first, the objection urged here against this medium as a mo- 

 tive power on the railway— rather, perhaps, as admitting a doubt, 

 than to be taken in the light of a substantial, well settled argu- 

 ment for unqualified rejection— is, " the fear of a practical difii- 

 culty from the rapid expenditure of power in propulsion, and the 

 great diminution of pressure in the last mile ; so as to render 

 necessary a variable cut-off, and an inconveniently large cylinder." 



This objection, even if valid, in whole or in part, will vanish, and 

 all doubt upon the subject be put at rest by considering the feasi- 

 bility of providing a self-acting regulator, and placing it between 

 the receiver and driving cylinders, through which the compressed 

 fluid shall be made to pass, and from which in such case, not 

 direct from the receiver, it will then be allowed to flow into the 

 cylinders, with such an amount of pressure as may be requisite ; 

 holding back, in the main receiver, the balance of the original 

 pressure, to be called into service as needed. So, if the working 

 pressure required for propulsion be twenty pounds on the square 

 inch, it will only be necessary, in the regulator, to preserve that 

 constant pressure, in order to provide for the steady supply of the 

 driving cylinders, leaving the stock in the receiver to be drawn 

 out by the regulator to supply its own exhaustion and to preserve 

 its own stock. So that the question of the validity of the pro- 

 posed objection, so far as it relates to the rapid expenditure of 

 power, need not be discussed ; seeing that any difficulty, of the 

 kind supposed, may be readily provided against by the use of the 

 regulator. 



It can be no very serious objection to the proposed system 

 that it may require the intervention of a cut-off", any more than 

 it is with the steam system. The advantages arising from it are 

 far greater in the one case, in the absence of heat and vapor as 

 a means of economizing the expenditure of power, than in the 

 other where those influences do not exist. All the writers, ex- 

 perimenters and analogies concur in this view. " The principle 

 of expansion," says Parsey, "can be carried out to perfection by 

 compressed air." It being an expansive power, always of the 

 natural temperature in the reservoir, as soon as it moves the pis- 

 ton its density must be equal to the resistance, and having no 

 disposition to condense (not being adulterated with heat or 

 vapor) its activity will follow the piston the whole length of the 



