HYDRAULICS. 19 i 



thing with a single pump ; and many different con- 

 structions for this purpose have been proposed and 

 executed. Fig. 5. represents one of the best. It 

 consists of a working-barrel, a b, closed at both 

 ends; the piston c is solid, and the piston-rod 

 passes through a collar of leathers at the top of 

 the barrel. This barrel communicates laterally 

 with two pipes, n and k, the communications being 

 as near to the top and bottom of the barrel as 

 possible. At each of the communications are two 

 valves, opening upwards. The two pipes unite in 

 a larger rising-pipe at b, which bends a little back, 

 to give room for the piston-rod. Suppose the pis- 

 ton down close to the entry of the lateral pipe h; 

 when it is drawn up, it compresses the air above it, 

 and drives it through the valve in the pipe k, 

 whence it escapes through the rising-pipe ; at the 

 same time it rarefies the air below it. Therefore, 

 the weight of the atmosphere shuts the valve m, 

 and causes the water in the cistern to rise through 

 the valve n, and fill the lower part of the pump. 

 When the piston is pushed down again, this water 

 is first driven through the valve m, because n im- 

 mediately shuts; and then most of the air which 

 was in this part of the pump at the beginning, 

 goes up through it, some of the water coming 

 back in its stead. In the mean time, the air which 

 remained in the upper part of the pump after the 

 ascent of the piston, is rarefied by its descent; 

 because the valve o shuts as soon as the piston 

 begins to descend, the valve p opens, the air in the 

 suction-pipe h expands into the barrel, and the 

 water rises into the pipes by the pressure of the 

 atmosphere. The next rise of the piston must 

 bring more water into the lower part of the barrel, 

 and must drive a little more air through the valve o, 



