a 



Machine and its Model. 269 



the same with that of the model, these parts, so far as this action is 



concerned, will be in keeping with each other. 



In the case, however, of a lateral support, the distance through 

 which it can yield without breaking is not augmented by an enlarge- 

 ment of the scale ; so that, in these parts, the large engine is com- 

 paratively weak, even although the velocity of the motion be the 

 same on the large as on the small scale. 



But those motions which are most likely to produce accidents 

 in this way, are generated by descents bearing a fixed proportion 

 to the dimension of the engine: the velocity, therefore, is generally 

 greater in the large engine than in the small one, so that large ma- 

 chines are more liable to accidents arising from the derangement of 

 any of their motions than small ones are : they possess, however* 

 more absolute strength, and are better able to resist any extraneous 

 force. We must carefully distinguish between the absolute strength 

 of any structure, or the power which it has of resisting impressions 

 from without, and the ability of that structure to withstand the effects 



of derangement among its own parts. 



Every one knows that a thermometer bulb is broken by a very 

 slight blow, and that yet it may fall from a considerable height with- 

 out injury. Yet a large ball, of a proportionate thickness, though 

 able to resist a much severer blow, is dashed to pieces by a fall* 

 The insect is crushed by a touch ; yet many species of insects pos- 

 sess the power of leaping to distances inconceivable, when compared 

 with the minuteness of the animal. 



Whether we consider its ability to resist mere pressure, or its 

 ability to resist an impulse, the performance of an engine is not at 

 all commensurate with that of its model. It remains for me to shew 

 that as great a disparity is perceived when we consider the friction 

 of the parts. As, perhaps I have been rather general in my previ- 

 ous statements, I shall, when speaking of the friction, confine my 

 attention to that very important instrument the steam-engine. A 

 little consideration will enable any one to apply similar remarks to 

 other machines. 



The steam-engnie moves on account of the pressure of the steam 

 against the surface of the piston ; which pressure may be estimated 

 at about ten pounds per circular inch. The friction which this pres- 

 sure has to overcome may be divided into three parts : the first in- 

 cluding all friction caused by the packing of the piston and stuffing- 

 boxes, and which is proportional to the linear dimension simply ; 



Vol. XXIV.— No. 2. 35 



