ON THE GENERAL THEORY OF THE STEAM ENGINE. 165 



Also the number of horses' power on two cylinders, according to Tredgold's general rule, page 

 197, is found to be, 



-a 2 P x 2 n I 



33000 



. .(F). 



33000 



The numerator of this expression represents the quantity of steam (IT n a 2 /) consumed per 

 minute multiplied into the pressure (P) at which it is supplied to the cylinder : and, the tem- 

 perature T being supposed to be preserved, this product, in fact, expresses the evaporating 

 power of the boiler, including, of course, the necessary reductions for the unavoidable losses 

 by the passage of the steam through the steam pipe, the clearance of the cylinder, &c. ; and 

 it hence also appears that the effective evaporating power of an engine is proportional to the 

 number of horses' power. 

 The equation (F) gives also, 



n p = 33000 H _ _ (G) 

 TT a 3 1 



Thus it appears, that when the engine is working at a given power, the number (n) of strokes 

 per minute will vary inversely as the mean pressure P on the piston, which is a principle 

 particularly applicable to the motion of Locomotive Engines, where the forces P, R, are 

 subject to considerable variations. 



The special relations of the motion of locomotive engines have been ably treated by the 

 Chevalier G. De Pambour, in a valuable treatise recently published by Mr. Weale, which 

 contains much valuable practical information on the subject. From numerous experiments 

 with different engines on the Liverpool and Manchester Railway, made under the author's 

 inspection, with the view of ascertaining the amount of friction, he has arrived at the following 

 useful conclusions, respecting the resistance to traction, which may be regarded as approxi- 

 mate estimations for locomotive engines as at present constructed, viz. 



1. That the average tractive friction of the carriages, without the engine and tender, may 

 be reckoned at 8 fts. per ton of the entire weight of the carriages and load. (Page 116). 



2. That the average friction or resistance to traction of well-constructed locomotive engines, 

 in good order, is about 15 fts. per ton of their weight. (Page 136). 



3. That the tractive friction of the engine is increased by the load at an average of about 1 ft. 

 per ton. (Page 157). 



We may hence add, 



4. That the friction of the wheels alone of an engine, by the first estimation, being 8 fts. 

 per ton of its own weight, the friction due to its own weight as a load being 1 ft. per ton, 

 and the whole resistance being 15 fts. per ton, it follows, that the friction of the engine gear 

 alone, independently of any connexion with the rails, is, on the average, about 6 fts. per ton 

 of the weight of the engine applied at the circumference of the wheel. 



5. That the entire resistance to an engine and train may be estimated at 9 fts. per ton on 

 the gross weight of the engine, tender, carriages, and load, increased by 6 fts. per ton on the 

 weight of the engine alone. 



