MECHANICS AND HYDROSTATICS. 99 



The experimental truth of this principle is a 

 matter of obvious and universal experience. The 

 weight of a basket of stones is not altered by shak- 

 ing the stones into new positions. We cannot 

 make the direct burden of a stone less by altering 

 its position in our hands ; and if we try the effect 

 on a balance or a machine of any kind, we shall 

 see still more clearly and exactly that the altered 

 position of one weight, or the altered arrangement 

 of several, produces no change in their effect, so 

 long as their point of support remains unchanged. 



This general fact is obvious, when we possess in 

 our minds the ideas which are requisite to appre- 

 hend it clearly. But when we are so prepared, the 

 truth appears to be manifest, even independent of 

 experience, and is seen to be a rule to which expe- 

 rience must conform. What then is the leading Idea 

 which thus enables us to reason effectively upon 

 mechanical subjects? By attention to the course 

 of such reasonings, we perceive that it is the Idea 

 of Pressure ; Pressure being conceived as a mea- 

 surable effect of heavy bodies at rest, distinguish- 

 able from all other effects, such as motion, change 

 of figure, and the like. It is not here necessary to 

 attempt to trace the history of this Idea in our 

 minds ; but it is certain that such an Idea may be 

 distinctly formed, and that upon it the whole 

 science of statics may be built. Pressure, load, 

 weight, are names by which this Idea is denoted 

 when the effect tends directly downwards; but we 



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