1 72 THE ART OF PROJECTING. 



diameter, the lens to be one foot focus, the distance to 

 the wall 25 feet. How large will the screen need to be 

 to receive the image ? Asi:2S::3:x=7S inches = 

 6 feet. Suppose the object to be one inch long. 

 What must be the focal length of a lens to project the 

 image 4 feet long when the screen is 20 feet away ? As 

 48 : i : : 240 : x = 5 inches. In this way one may pro- 

 vide distance, screen, and lenses to suit his con- 

 venience near enough for all practical purposes. 



VORTEX RINGS AND THEIR PHENOMENA. 



The phenomena presented by vortex rings are so 

 interesting some of them so surprising and yet are 

 so easily produced as to warrant giving some space to 

 an account of their production. Aside from this the 

 physical importance of their study is very great, seeing 

 that Sir Wm. Thomson and others have seriously 

 proposed to account for the properties of atoms of mat- 

 ter by supposing the latter to be vortex rings of ether in 

 the ether. The ring of steam often seen puffed from 

 a locomotive, and rising in the air sometimes a hundred 

 feet or more, is called a vortex ring or sometimes 

 a smoke ring. Such rings are formed whenever a gas 

 or a liquid is suddenly pushed through an orifice. If 

 they be formed solely of air they cannot be seen, on 

 account of their transparency, but they may be made 

 manifest in other ways. 



Over the mouth of a glass or a tin funnel three or 

 four inches in diameter, tie a piece of stout paper. 

 Snap with the finger upon this stretched paper and a 

 ring will be projected from the stem. If the latter 

 be directed towards the face it will be easily felt, and 

 if it be directed towards a candle flame it may blow it 



