264 On the relation which subsists between a 



Art. VII. — A few remarks on , the relation which subsists between 

 a Machine and its Model; by Edward Sang, Teacher of Math- 

 ematics, Edingburgh. Communicated by the Author. 



From the Edinburgh New Philosophical Journal. 



At first sight, a well constructed model presents a perfect repre- 

 sentation of the disposition and proportion of the parts of a machine, 

 and of their mode of action. 



Misled by the alluring appearance, one is apt, without entering 

 minutely into the inquiry, also to suppose that the performance of 

 a model is, in all cases, commensurate with that of the machine 

 which it is formed to represent. Ignorant of the inaccuracy of such 

 an idea, too many of our ablest mechanicians and best workmen, 

 waste their time and their abilities on contrivances which, though they 

 perform well on the small scale, must, from their very nature, fail 

 when enlarged. Were such people acquainted with the mode of 

 computing the effects, or had they a knowledge of natural philoso- 

 phy, sufficient to enable them to understand the basis on which such 

 calculations are founded, we should see fewer crude and impractica- 

 ble schemes prematurely thrust upon the attention of the public. This 

 knowledge, however, they are too apt to regard as unimportant, or as 

 difficult of attainment. They are startled by the absurd distinction 

 which has been drawn between theory and practice, as if theory were 

 other than a digest of the results of experience; or, if they overcome 

 this prejudice, and resolve to dive into the arcana of philosophy, they are 

 bewildered among names and signs, having begun the subject at the 

 wrong end. That the attainment of such knowledge is attended 

 with difficulty is certain, but it is with such difficulty only as can be 

 overcome by properly directed application. It would be, indeed, 

 preparing disappointment, to buoy them up with the idea, that knowl- 

 edge, even of the most trivial importance, can be acquired without 

 labor. Yet it may not be altogether unuseful, for the sake both of 

 those who are already, and of those who are not, acquainted with 

 these principles, to point out the more prominent causes, on account 

 of which the performance of no model can, on any occasion, be con- 

 sidered as representative of that of the machine. Such a notice will 

 have the effect of directing the attention, at least to this important 

 subject. In the present state of the arts, the expense of constructing 



