396 Intelligence, fyc. 



bring the two cards towards each other — when within f of 

 an inch, if the current of air be strong, the loose card will 

 suddenly rise, and adhere to the perforated card. If the 

 card through which the tube passes, have several perforations 

 made in it, the loose card is instantly thrown off by a slight 

 puff of air. An explanation is requested by your ob't. serv't., 



Nat. Ives. 



Explanation by Dr. Robert Hare. — The phenomenon above al- 

 luded to, is usually illustrated by means of two disks,* into the 

 centre of one of which a tube is fastened, so that on blowing 

 through the tube the current is arrested by the moveable disk. 

 Under these circumstances the moveable disk is not removed, as 

 would be naturally expected. 



Supposing the diameter of the disks to 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 disks were to be separated (their surfaces remain- 

 ing parallel) with a velocity as great as that of the blast, a col- 

 umn of air must meanwhile be interposed, sixty-four times great- 

 er than that which would escape from the tube during the inte- 

 rim. Consequently if all the air necessary to preserve the equil- 

 librum be supplied from the tube, the disks 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 cannot be supplied from any other source, unless 

 a deficit of pressure be created between the disks unfavorable 

 to their separation. 



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

 disks cannot be made to move asunder with a velocity greater 

 than l-64th 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 disk, and the thin ring 

 of air which exists around the orifice between the disks ; and 

 since the moveable disk can only move with l-64th of the velo- 

 city of the blast, the ring of air in the interstice must experi- 

 ence nearly all the momentum of the jet ; and must be driven out- 

 wards, the blast following it in various currents, radiating from 

 the common centre of the tube and disks. 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, Vol. 11. p. 172; 



* The word disk is used by experimental philosophers, to signify any plane 

 surface bounded by a circle, whether it be merely a superficies, or have a 

 sensible thickness, as in the case of a wafer, or a piece of coin. 



