MECHANICS AND HYDROSTATICS. 97 



same effect as if they were both together supported immediately at 

 that point. Or more generally, we may state the principle to be this : 

 that the pressure by which a heavy body is supported continues the 

 same, however we alter the form or position of the body, so long AS 

 the magnitude and material continue the same. 



O 



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 shaking 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 idea^ 

 which are requisite to apprehend it clearly. But when we are so pre- 

 pared, the truth appears to be manifest, even independent of experience, 

 and is seen to be a rule to which experience 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 reason- 

 ings, we perceive that it is the idea of Pressure ; Pressure being con- 

 ceived as a measurable effect of heavy bodies at rest, distinguishable 

 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, iveiyht, are names by which this idea is denoted when the effect 

 tends directly downwards ; but we may have pressure without motion, 

 or dead 'pull, in other cases, as at the critical instant when two nicely- 

 matched wrestlers are balanced by the exertion of the utmost strength 

 of each. 



Pressure in any direction may thus exist without any motion what- 

 ever. But the causes which produce such pressure are capable of pro- 

 ducing motion, and are generally seen producing motion, as in the 

 above instance of the wrestlers, or in a pair of scales employed in 

 weighing ; and thus men come to consider pressure as the exception, 

 and motion as the rule : or perhaps they image to themselves the mo- 

 tion which might or would take place ; for instance, the motion which 

 the arms of a lever would have if they did move. They turn away 

 from the case really before them, which is that of bodies at rest, and 

 balancing each other, and pass to another case, which is arbitrarily 

 VOL. I. 7 



