

EFFLUX OF GASES 303 



reaction- wheel, which is driven by steam, will be de- 

 scribed hereafter. 



Screw-propellers act in air as well as in water. The 

 sails of a windmill are applications of the principle on 

 which the action of the screw depends, on a larger 

 scale ; on a small scale the same principle is applied in 

 the toy-mill, which is set in motion by presenting its 

 sails, made of wood or feathers, to a current of air. Fig. 

 200 A represents a spiral cut out of paper, and balanced 

 upon a knitting needle. If an ascending current of air 

 is produced by placing a spirit-flame underneath it, the 

 spiral begins to rotate, precisely in the same manner as 

 the screw opposed to a current of water, which was 

 shown on page 202, fig. 145. 



Fig. 200 J? indicates how the spiral is drawn upon a piece of stiff 

 paper. From the centre a describe the semicircle 5 c d ; from b 

 the semicircle def, and again from a describe fgh, and so on. 

 With the blunt point of a knitting needle make a shallow cavity in 

 the paper, between a and 6, and cut the spiral. Fix the knitting 

 needle in a hole made in the top of the retort-stand with the bradawl, 

 place the cavity of the spiral upon the point of the knitting needle, 

 and the spiral will assume by its weight the form shown in fig. 

 200 J. 



When a light wheel provided with blades, like those 

 of a screw-propeller, is set in rapid rotatory motion, and 

 is not fixed, it will not only have a progressive motion, 

 shut it will even ascend in the air. The 4 flying tops/ 

 and 'boomerang tops' sold in toy-shops, are applications 

 3f this principle ; the wheel in most of them may be set in 

 j i'apid motion by means of a spring which acts within a 

 ;ase ; as soon as the little wheel leaves the spring, it darts 

 )ff into the air, and continues to rotate for some time by 

 ts inertia. Another kind of flying top is made to 



