104 Mr Stewart on the Causes of Obstruction in 



vacancy in the upper part will, as in the case of the vertical tube, 

 be in a short lime occupied by the rarefied air which evolves it- 

 self from the water in consequence of the removal of the atmo- 

 spherical pressure ; and if, after this has taken place, the syphon 

 be once more lowered to its original depth in the reservoir, the 

 water will indeed again rise until the rarefied air above it is com- 

 pressed into the same density with that of the atmosphere ; but 

 the air will maintain its place in the upper part, and the flow of 

 the water will not be resumed. This evolution of air in the 

 syphon occurs not merely when the diminution of atmospherical 

 pressure approaches its extreme limit, but more or less in all 

 circumstances. For, at the summit or vertex of the syphon, 

 there is not only a certain diminution of pressure, even when its 

 elevation is small, but the gravity of the descending column in 

 the longer limb tends to weaken the cohesion of the fluid, and 

 to bring into play another co-operating cause of gaseous develop- 

 ment, which shall presently be explained. So that, in this in- 

 strument, the causes of accumulating obstruction are constantly 

 at work, lessening its utility, and greatly limiting the applica- 

 tion of which it is really susceptible to various useful purposes. 

 It has been stated in the outset, that the capacity of fluids for 

 retaining air in solution depends not only on the amount of at- 

 mospherical pressure, but on the varying degree of the corpus- 

 cular attraction of its particles ; and every cause which weakens 

 that attraction tends to the disengagement of the combined air. 

 The expansion of fluids by heat is an agent of this description, 

 and as it is one of convenient apphcation, it is accordingly em- 

 ployed for this purpose, both in chemistry and the arts, and par- 

 ticularly for expelling the air from quicksilver in the construc- 

 tion of the barometrical tube. In the case just detailed, both 

 of these causes appear to be in operation. Foi*, while the sus- 

 taining atmospherical pressure operates with less and less force 

 on each successive film of the ascending column of water, the 

 counteracting force of gravitation is, practically, the same on all, 

 operating with the same intensity on the highest as on the lowest 

 film ; — and it acts with still greater and more undivided force 

 on the descending column in the longer limb, in a manner yet 

 more direct and palpable, causing the fluid to descend with a 

 force continually accelerating, while it is ascending in the lower 



