566 



THE STEAM-ENGINE. 



ments, 3-36 cubic feet of water produced 1,722 cubic feet of steam, of the 

 density worked in the cylinders. The ratio, therefore, of the volume of this 

 steam to that of the water producing it, was 1,722 to 3-36, or 512-5 to 1. 

 The pressure of steam of this density would be 54-5 pounds per square inch. 

 Such, therefore, was the limit of the average total pressure of the steam in the 

 cylinders. In this experiment the safety-valve of the boiler was screwed 

 down to 60 pounds per square inch above the atmospheric pressure, which 

 was therefore the major limit of the pressure of steam in the boiler ; but as 

 the actual pressure in the boiler must have been less than this amount, the 

 difference between the pressure in the cylinder and boiler could not be ascer- 

 tained. This difference, however, would produce no effect on the moving 

 power of the steam, since the pressure of steam in the cylinders obtained by 

 the above calculation is quite independent of the pressure in the boiler, or of 

 any source of error except what might arise from priming. The pressure of 

 54-5 pounds per square inch, calculated above, being the total pressure of the 

 steam on the pistons, let 14-5 pounds be deducted from it, to represent the 

 atmospheric pressure against which the piston must act, and the remaining 40 

 pounds per square inch will represent the whole available force drawing the 

 train and overcoming all the resistances arising from the machinery of the 

 engine, including that of the blast-pipe. The magnitude of a 12^ inch piston 

 being 122'7 square inches, the total area of the two pistons would be 245*2 

 square inches, and the pressure upon each of 40 pounds per inch would give 

 a total force of 9,816 on the two pistons. Since this force must act through a 

 space of three feet, while the train is impelled through a space of 15-7 feet, 

 it must be reduced in the proportion 3 to 15-7, to obtain its effect at the point 

 of contact of the wheels upon the rails : this will give 1,875 pounds as the 

 total force exerted in the direction of the motion of the train. The gross 

 weight of the train being 80 tons, including the engine and tender, this would 

 give a gross moving force along the road of about 23-4 pounds per ton of the 

 gross load, this force being understood to include all the resistances due to the 

 engine. This resistance corresponds to the gravitation of a plane rising at the 

 rate of -^, and therefore it appears that such would be the inclination of the 

 plane by the gravitation of which the gross resistance would be doubled, in- 

 stead of such inclination being about -g-i^, as has been hitherto supposed. 



Since the remarkable and unexpected results of this series of experiments 

 became known various circumstances were brought to light, which were be- 

 fore unnoticed, and which abundantly confirm them. Among these may be 

 mentioned the fact, that in descending the Madeley plane, on the grand junc- 

 tion railway, which falls for above three miles at the rate of twenty-nine feet 

 a mile, the steam can never be entirely cut off. But, on the other hand, to 

 maintain the necessary speed in descending, the power of the engine is always 

 necessary. As this plane greatly exceeds that which would be sufficient to 

 cause the free motion of the train down it, the power of the engine expended 

 in descending it, besides all that part of the gravitating power of the plane which 

 exceeds the resistance due to friction and other mechanical causes must be 

 worked against the atmosphere. 



This estimate of the resistance is also in conformity with the results of a 

 variety of experiments made by me with trains of different magnitudes down 

 inclined planes of various inclinations. 



In laying out a line of railway the disposition of the gradients should be 

 such as to preserve among them as uniform a character as is practicable, for 

 the weight and power of the engine must necessarily be regulated by the 

 general steepness of the gradients. Thus if upon a railway which is generally 

 level, like that between Liverpool and Manchester, one or two inclined planes 



