36 Experimental Philosophy. [Lecture 4. 



its surface like drops of dew. From this expe- 

 riment it may be inferred, that if water is indeed 

 capable of compression, it is so only in a very 

 slight degree, since, instead of yielding to the 

 force of pressure, it found its way out through 

 the pores of the metal. The same has been 

 proved more scientifically by subsequent philo- 

 sophers. 



The first principle that may be laid down with 

 respect to the pressure of fluids is, that the sur- 

 face of all waters which have a communication 

 whilst they are at rest will be perfectly level. 

 To explain this more fully, observe the three 

 united tubes (Plate III. fig. 7). It will be seen 

 that if water is poured into the perpendicular 

 tube A, it will run through the horizontal tube 

 C, and rise in the opposite perpendicular tube B 

 to the same height at which it stands in A. 



Hence appears the reason why water, con- 

 veyed under the earth through conduit-pipes, 

 will always rise to the level of the reservoir 

 whence it is drawn. It is in this manner that 

 the cities of London and Westminster are sup- 

 plied with water, either from London Bridge 

 water-works or the New River. In the former 

 case, water is raised from the Thames by immense 

 pumps worked by wheels, which are turned by 

 the tide, to the highest part of the town whither 

 water is to be conveyed by pipes ; and, in the 

 latter, it is well known that the reservoir of the 

 New River stands on a rising ground near Isling- 



