ON THE GENERAL THEORY OF THE STEAM ENGINE. 187 



have every reason to suppose that they were all of them approximate estimations for the work 

 of such engines as Mr. Tredgold had access to for his experience, which, in reference to engines 

 now constructed, were no doubt of very limited dimensions and power, and of comparatively 

 inferior workmanship. There can be no doubt that estimations of this kind, where a fixed 

 proportion of the whole power is taken, are not to be depended upon. It cannot, however, 

 be overlooked that M. Pambour, in throwing discredit on what he terms the ordinary theory, 

 has unfairly applied the principle of constant coefficients in such a way as had never been 

 contemplated by Tredgold or any one else ; besides, it should be observed, that M. Pambour 

 has only evaded the difficulty in another way : he has estimated the friction and losses in a 

 manner more in accordance with the nature of the steam engine, but, nevertheless, equally as 

 empirical as that of Tredgold ; and the state of the problem remains nearly the same as it did 

 before. For instance, in his Treatise on Locomotive Engines, page 184, he takes the effective 

 evaporation through the cylinders at three-fourths of the total evaporation in the boiler, viz., 

 the effective evaporating power O3 cubic foot, and the total evaporating power 0'4 cubic foot 

 of water, for each square foot of surface exposed to the fire. On the whole, we are compelled 

 to conclude that such estimations are not to be relied upon as general rules ; that where 

 accuracy is really wanted, the actual amount of loss, &c., is only to be estimated by having 

 recourse to the experience of each individual description of engine under given circumstances ; 

 and that the correct value of the effective power of an engine is only to be found in the mean 

 pressure on the piston, and the velocity with which it moves, according to the old rule. 



There is one more point in M. Tambour's strictures on Tredgold's principles which it may 

 be necessary to notice, being one on which he has, by some sort of inattention, allowed 

 himself to be grossly deceived. The point in question, which, on being misrepresented, might 

 possibly mislead others, is involved in the following extract from page 9 of his Memoir on the 

 Steam Engine : 



" Tredgold, in his Treatise on Steam Engines (Art. 127 and following), undertakes to 

 " calculate the velocity of the piston from considerations deduced from the velocity of the 

 " flowing of a gas, supposed under a pressure equal to that of the boiler, into a gas supposed 

 " at the pressure of the resistance. He concludes from thence, that the velocity of the piston 

 " would be expressed by this formula, 



V = 6-5 \Th, 



" in which V is the velocity in feet per second, and h stands for the difference between the heights 

 " of two homogeneous columns of vapour, one representing the pressure in the boiler, the other 

 " that of the resistance. But it is easily seen, that this calculation supposes the boiler filled with 

 " an inexhaustible quantity of vapour, since the effluent gas is supposed to rush into the other 

 " with all the velocity it is susceptible of acquiring, in consequence of the difference of pressure. 

 " Now such an effect cannot be produced, unless the boiler be capable of supplying the expendi- 

 " ture, however enormous it might be. This amounts consequently to supposing that the produc- 

 " tion of steam in the boiler is unlimited. But, in reality, this is far from being the case. It is 

 " evident that the velocity of the piston will soon be limited by the quantity of steam pro- 

 " ducible by the boiler in a minute. If that production suffice to fill the cylinder 200 times 

 " in a minute, there will be 200 strokes of the piston per minute ; if it suffice to fill it 300 

 " times, there will be 300 strokes. It is then the vaporization of the boiler which must 



