Plane Surfaces by a Blast, in certain Cases, 95 



be to that of the orifice as 8 to 1, the area of the former to the 

 latter must be as 64? to 1. Hence if the discs were to be se- 

 parated (their surfaces remaining parallel) with a velocity as 

 great as that of the blast, a column of air must meanwhile be 

 interposed 64? times greater than that which would escape 

 from the tube during the interim. Consequently, if all the air 

 necessary to preserve the equilibrium be supplied from the 

 tube, the discs must be separated with a velocity as much less 

 than that of the blast, as the column, required between them, 

 is greater than that yielded by the tube ; and yet the air can- 

 not be supplied from any other source, unless a deficit of pres- 

 sure be created between the discs, unfavourable to their se- 

 paration. 



It follows, then, that under the circumstances in question, the 

 discs cannot be made to move asunder with a velocity greater 

 than l-64?th of that of the blast. Of course all the momentum 

 of the aerial particles which constitute the current through 

 the tube, will be expended on the moveable disc, and the thin 

 ring of air which exists around the orifice between the discs ; 

 and since the moveable disc can only move with l-64th of the 

 velocity of the blast, the ring of air in the interstice must ex- 

 perience nearly all the momentum of the jet, and must be 

 driven outwards ; the blast following it in various currents ra- 

 diating from the common centre of the tube and discs. The 

 effect of such currents in producing an afflux of the adjoining 

 portions of any fluid in which they may be excited, is well 

 known, having been successfully illustrated by Venturi. (See 

 Nicholson's Journal, quarto series, vol. ii. p. 172.) Accordingly 

 the afflux of air towards the discs counteracts the small velo- 

 city which the blast would communicate, and thus prevents 

 their separation, and may even cause them to approach each 

 other, if previously situated a small distance apart. 



This rationale commences with the assumption, that the 

 discs will remain nearly parallel. That there cannot be much 

 deviation from parallelism must be evident ; since any obliquity 

 will make the opening greater on one side than on the other ; 

 and the jet proceeding with most force towards the widest 

 opening, will increase the afflux of air upon the outer surface 

 of the moveable disc in the part where the current is strongest, 

 and thus correct the obliquity. 



XV. Chemical 



