62 Perkins'' Steam Engine. 



not taken into view in this diagram. As the connecting rod 

 is intended to be four diameters of the path of the crank, 

 the variation will make no practical objection, being, at its 

 greatest value, but one-thirty-second part of its range. If 

 the engine should be worked by a connecting rod, as is 

 sometimes the case in steam boats, say only one diameter of 

 the path of the crank, the variation at each end of the 

 stroke, would amount to a practical defect, since the piston 

 would move with nearly three times the velocity at the low- 

 est quarter of the stroke, that it would at the first quarter. 

 Thus circumstanced, the crank must be above the cylinder. 



As the law of expansion seems not yet to be settled, an 

 arithmetical expansion has been used for this diagram, which, 

 from its approximation to the real law, will be quite near 

 enough for practical purposes. Many who are of the school 

 of Tillock and Wolf, believe that the expansive power of 

 steam depends upon heat only ; while the Soho experiments 

 are said to prove that elasticity depends simply on density, 

 without regarding temperature, viz., that if a cubit foot of 

 steam at atmospheric pressure, weighs one ounce, 50 at- 

 mospheres of steam would weigh 50 ounces ; but Dalton, 

 who is undoubtedly much nearer the true law, would make 

 50 atmospheres weigh but about 34 ounces. 



I have no doubt that the nearer the atoms of water are made 

 to approach each other, by compression, the greater will be 

 the repulsive action of caloric, and that, in a more rapid ra- 

 tio than has hitherto been allowed, especially in highly com- 

 pressed steam. Its comparative density with the increase of 

 power, diminishes faster than has been supposed even by 

 Dalton. 



6. Perkins'' Steam Engine. 



We have seen this engine repeatedly in action since our last 

 notice of it, and to all appearance giving great satisfaction to 

 those who have visited it ; there has not, however, yet been 

 exhibited any demonstration of the actual amount of power 

 which it is capable of exerting, nor do we consider that its 

 present situation, in Mr. Perkins' factory, is at all favorable 

 to such an experiment. The public must, therefore, for the 

 present, be satisfied with such inferencial proofs, of its ca- 

 pabilities, as may be drawn from a consideration of the 

 amount of friction exerted upon the fly wheel, by the weight- 

 ed lever described in our former report. 



