ELECTRIC LIGHTS. 



'73 



out at a distance of three or four feet. With a larger 

 funnel and stronger blow it may extinguish the flame at 

 a greater distance. To make them visible it is neces- 

 sary to mix the fumes of something, smoke for instance, 

 with the air of which they are formed. Very good 

 ones may be formed by" the mouth. Let the mouth 

 be filled with smoke and the lips be pursed as if to 

 produce the vowel 0, then tap the cheek with the end of 

 the finger, and smoke rings an inch or two in 

 diameter will be formed. Some smokers are able 

 to project very large and dense ones from their 

 mouth by a sudden forward thrust of the back of 

 the mouth, a movement which has to be acquired by 

 practice. 



For the production of vortex rings for the study of 

 their behavior, it will be necessary to have made a 

 box which may be kept filled with the visible vapor. 

 One of the following shape and dimensions will be found 

 to answer well. A box of wood 

 about a foot cube, having a 

 round hole about four inches in 

 diameter cut in the middle of 

 one side. A swinging hinged 

 back, framed square, over which 

 may be stretched tightly some 

 stout cotton cloth, will close 

 the box tightly enough when it 

 is down. The cut represents the back of the box 

 lifted up. It will be convenient to have two strips 

 in front grooved so as to permit a slide to be inserted. 

 Several slides may be made for this position, each one 

 having its orifices through which the smoke may be pro- 

 jected. The following are suggested as being useful. 

 One with a round hole three inches in diameter. One 



