158 The Earl of Berkeley and Dr. Burton on a 



The semi-automatic valve. — This is shown in fig. 1 at S, and 

 in fig. 5 in section. It is used to shut off the pump from the 



To Dead weight gauges 



rest of the pressure-system when the plunger has been pushed 

 in to its limit, and has therefore to be run out again so as to 

 draw a fresh lot of oil into the pump. This valve must 

 accordingly be efficient up to the highest pressures used 

 (1500 atmospheres), and we have not found needle-valves of 

 the ordinary type satisfactory. The connexions to the pump 

 and to the pressure-tubes are as indicated ; and in the view 

 shown there is free communication between them because 

 the valve W is withdrawn from its seat. The valve-face is 

 formed by a ring of dermatine, sectioned in black. The 

 slow leakage of oil which would otherwise take place around 

 the unpacked valve-stem S is prevented by the auxiliary 

 needle-valve N. It is only while the valve is being altered 



