m'r>ft ! dnri 



REACTION- WHEELS. 201 



pipe ; during this operation the side of the cork which is turned 

 downwards in the figure must evidently 

 be turned upwards, and the sealing-wax 

 must be allowed to cool before the appa- 

 ratus is set to work. For this purpose a 

 hollow is made with the centre-punch in 

 a small piece of sheet-metal for the recep- 

 tion of the steel point ; two holes are also 

 drilled through it, so that it may be fixed 

 by two screws to the foot of the retort- 

 stand. Finally, a piece of wire of such 

 thickness that it may easily turn in the 

 hole of the brass strip at the top of the 

 cylinder is bent at right angles and 

 clamped as shown in the figure ; the por- 

 tion of the wire which is bent down- 

 wards should be slightly oiled, a drop of 

 oil should also be placed in the hollow 

 made for the pivot. The whole is best 

 placed in a small tub, to prevent the 

 splashing about of the water. The cylin- FIG. 144 (an. proj. real size). 

 der may be filled with water from a jug 



with a spout, and kept nearly full for some time. It will immediately 

 begin to rotate, and if it be provided with four orifices instead of two 

 the motion will be still more rapid. 



A wheel, formed of several pieces (4 or 6), which are 

 placed in an oblique direction upon the axis, as shown 

 in fig. 145, may be considered as a portion of a 

 screw, which has as many threads as there are pieces 

 or 'blades.' If such a wheel is made to rotate in 

 water, it will also move forwards in the water, be- 

 cause the water, which acts the part of the nut in 

 this case, tends to remain at rest by its inertia ; thus, 

 if the wheel were turned in the direction of the arrow 

 rf, it would at the same time have a progressive motion 

 in the direction of the arrow/. Wheels constructed 

 on this principle are briefly called screws, and, as is 

 well known, are used for propelling steamships. 



