COMMON THINGS PUMPS. 



weight to sustain. It lias also to sustain the weight of the water 

 which is above it. The atmospheric pressure acting also on the water 

 in the well, is transmitted by the water to the bottom of the piston; 

 but this effect is diminished by the weight of the column of water 

 between the surface of the water in the well and the bottom of the 

 piston, for the atmospheric pressure must, in the first place, 

 sustain that column, and can only act upon the bottom of the; 

 piston in the upward direction with that amount of force by which, 

 it exceeds the weight of the column of water between the piston 

 and the well. The effect, therefore, on the piston is the same as 

 if it were pressed downwards by the weight of the column of 

 water between the piston and the well, and at the same time 

 pressed upwards by the atmospheric pressure. Thus the piston 

 may, in fact, be regarded as being urged downwards by the 

 following forces, the atmospheric pressure, the weight of the 

 water above the piston, and the weight of the water between the 

 piston and the well ; that is to say, in fact, by the atmospheric 

 pressure, together with the weight of all the water which has been 

 raised from the well. At the same time, it is pressed upwards by 

 the atmospheric pressure transmitted from the surface of the water 

 in the well. This upward pressure will destroy the effect of the 

 same atmospheric pressure acting downwards on the surface of 

 the water above the piston, and the effective downward force will 

 be the weight of all the water which is contained in the pump. 



By this reasoning, it appears that the pump must be worked 

 with as much force as is equal to the weight of 'all the water 

 which is in it at any time, and, therefore, that the atmospheric 

 pressure affords no aid to the working power. 



Since the action of the pump in raising water is subject to inter- 

 mission, the stream discharged from the spout will necessarily flow 

 by fits and irregularly, if some means be not adopted to prevent this. 

 At the top of the pump a cistern may be constructed, as shown in 

 g. 12, with a view to remove this inconvenience. If the pump be 

 worked, in the first instance, so as to raise more water in a given 

 time than is discharged at the spout, the column of water will 

 necessarily accumulate in the barrel of the pump above the spout. 

 The cistern Ji N will, therefore, be filled, and this will continue 

 until the elevation of the surface of the water in the cistern above 

 the spout will produce such a pressure, that the velocity of 

 discharge from the spout will be equal to the velocity with which 

 the water is raised by the piston. The level of the water in the 

 cistern will therefore cease to rise. This level, however, will be 

 -subject to a small variation as the piston rises; for while the 

 piston is descending, the water is flowing from the spout, and no 

 water is raised by the piston, consequently the level of the water 

 186 



