372 On the relation between a Machine and its Model. 



At the surface of Jupiter a steam-engine of twenty horses' power 

 would be unable to move : at the surface of our Earth, one of pre- 

 haps 1000 horses' power might perform pretty well ; but at the sur- 

 face of the Moon they might be made of perhaps 20.000 horses' 

 power, — supposing the pressures of the atmosphere in the three cases 

 to be alike. On Jupiter a steam-carriage would be an absolute 

 chimera ; on the earth it is barely possible ; but on the moon nothing 

 would be more feasible. An intensity of gravitation slightly greater 

 than that which the earth exerts, would altogether preclude the 

 hope of obtaining a locomotive engine. As it is, on flat rail-roads 

 they perform well ; as the road becomes inclined, they become less 

 practicable ; and, on common roads, nothing but the most consummate 

 skill in the selection and in the use of the material, as well as in the 

 contrivance of the parts, can ever be successful in their construction. 

 Security demands strength, strength requires weight, weight increases 

 friction, friction calls for additional power, and power can be pro- 

 cured only by an increase of weight. To reconcile these con- 

 flicting claims is not the task for a beginner in mechanical contrivance, 

 but for one well versed alike in the theory and in the practice of the 

 arts. Models are of no use, for, although the model be able to climb 

 a considerable ascent, that fact is no guarentee that a full-sized in- 

 strument will be able to follow its prototype. Let those who specu- 

 late on this matter remember that the elephant inhabits the plains, 

 and leaves the mountains to be tenanted by the smaller tribes ; and 

 let them also recollect, for the fact bears more upon the subject than 

 at first may appear, that the larger animals are most easily extermina- 

 ted ; that we have the fox and the rat, though the wolf be long since 

 gone. 



In the remarks which I have made, it has been my wish to place 

 the subject in such a light as might enable all to perceive the impor- 

 tance of its bearings ; and I have refrained from being practical, lest 

 in making myself better understood by some, 1 had rendered my 

 meaning obscure to others. My intention throughout has been to 

 inculcate the important truth, that no machine ever can be enlarged 

 QT (fiminished in proportion. 



