156 farmers' A^D mechanics' journal. 



When the edges of the two pieces of card were made to fit each 

 other very accurately, the upper card would be moved, and some^ 

 times it would be thrown oflP, but when the edges of the card, were 

 on two sides, sufficiently far apart to permit the current of air to 

 escape, the loose card retained its position, when the current of air 

 sent against it was strong, when it was inclined at every angle 

 through 180° ; but when very little inclined, if the current of air 

 ceased, the upper card would immediately fall. The experiment 

 succeeds equally well, whether the current of air be made by the 

 mouth, or from a bellows. When the tube fitted the perforation 

 of the card rather loosely, a comparatively light puff of air would 

 throw both cards three or four feet in height. When, from the 

 humidity of the breath, the upper surface of the perforated card 

 had a little expanded, and the two opposite sides were somewhat 

 depressed ; these depressed sides were distinctly seen to rise and 

 approach the upper card directly in proportion to the force of the 

 current of air. 



I have this moment discovered another fact with this simple ap- 

 paratus, equally inexplicable with the former. Let the loose card 

 be laid upon the hand, with the concave side up — blow forcibly 

 through the tube, and at the same time bring the two cards to- 

 wards each other — when within 3-8ths 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 otf by a slight puff of air. An explanation is requested 

 by your ob't. serv't., Nat. Ives. 



Explanation hy 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 oiifice 

 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 remaining 

 parallel) with a velocity as great as that of the bhst, a column of 

 air must meanwhile be interposed, sixty-four times greater than 

 that which would escape from the tube during the interim. Con- 

 sequently if all the air necessary to preserve the equilibrium 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 lube ; and yet the air can- 

 not 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 



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

 plane surface bounded hy a circle, whether it be merely a superficies, or 

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



