MECHANICS AND HYDROSTATICS. 99 



The difficulty of holding fast this Idea of Fluidity so as to trace 

 its consequences with infallible strictness of demonstration, may be 

 judged of from the circumstance that, even at the present day, men of 

 great talents, not unfamiliar with the subject, sometimes admit into 

 their reasonings an oversight or fallacy with regard to this very point. 

 The importance of the Idea when clearly apprehended and securely 

 held, may be judged of from this, that the whole science of Hydro- 

 statics in its most modern form is only the development of the Idea. 

 And what kind of attempts at science would be made by persons 

 destitute of this Idea, we may see in the speculations of Aristotle con- 

 cerning light and heavy bodies, which we have already quoted ; where, 

 by considering light and heavy as opposite qualities, residing in things 

 themselves, and by an inability to apprehend the effect of surround- 

 ing fluids in supporting bodies, the subject was made a mass of false 

 or frivolous assertions, which the utmost ingenuity could not reconcile 

 with facts, and could still less deduce from the asserted doctrines any 

 new practical truths. 



In the case of Statics and Hydrostatics, the most important condi- 

 tion of their advance was undoubtedly the distinct apprehension of 

 these two appropriate Ideas — Statical Pressure, and Hrjdrostatical 

 Pressure as included in the idea of Fluidity. For the Ideas being once 

 clearly possessed, the experimental laws which they served to express 

 (that the whole pressure of a body downwards was always the same ; 

 and that water, and the like, were fluids according to the above idea 

 of fluidity), were c o obvious, that there was no doubt nor difficulty 

 about them. These two ideas lie at the root of all mechanical science ; 

 and the firm possession of them is, to this day, the first requisite for a 

 student of the subject. After being clearly awakened in the mind of 

 Archimedes, these ideas slept for many centuries, till they were again 

 called up in Galileo, and more remarkably in Stevinus. This time, 

 they were not destined again to slumber ; and the results of their 

 activity have been the formation of two Sciences, which are as certain 

 and severe in their demonstrations as geometry itself, and as copious 

 and interesting in their conclusions ; but which, besides this recom- 

 mendation, possess one of a different order, — that they exhibit the 

 exact impress of the laws of the physical world, and unfold a portion 

 of the rules according to which the phenomena of nature take place, 

 and must take place, till nature herself shall alter. 



