between a Machine and its Model. 151 



largement of the scale ; so that, in these parts, the large engine 

 is comparatively 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 propor- 

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

 generally greater in the large engine than in the small one, so 

 that large machines 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 be- 

 tween 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 without 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 possess the power of leaping to distances in- 

 conceivable, 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 gene- 

 ral in my previous 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-engine 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 pressure has to overcome may be divided into three 

 parts : the first including all friction caused by the packing of 

 the piston and stuffing-boxes, and which is proportional to the 

 linear dimension simply ; the second including that part of tltp 

 friction on the gudgeons which arises from the pressure of tlie 



