86 



OPTICAL PROJECTION 



full when received, and weigh it so. Then the decrease in 

 weight in ounces multiplied by 0'7 will give us the amount of 

 gas used or escaped, and of course tell us what is left. Any 

 pair of scales that will weigh within half an ounce, is suffici- 

 ently accurate for this simple method of estimation. 



46. Pressure Kegulators. To use two bottles of com- 

 pressed gas in comfort, however, with only the one usual 



assistant, it was absolutely 

 necessary to deliver it to 

 the jets at approximately 

 constant pressure auto- 

 matically. Many attempts 

 were vainly made in this 

 direction, both in England 

 and America. It is per- 

 fectly easy to maintain a 

 constant pressure at the 

 outlet, whatever the pres- 

 sure behind, so long as an 

 outflow is kept up unob- 

 structed. But all early 

 contrivances simply kept 

 up a constant difference 

 of pressure between the 

 two sides of the regulating 

 valve, and directly the 

 outlet was turned off (as 

 at the jet) the pressure 

 rapidly accumulated be- 

 hind till the rubber tubes 



blew off. It was necessary to contrive that when pressure 

 thus began to accumulate, it should totally close the supply 

 from the cylinder, and take the strain off the frail rubber 

 tubes. This has now been accomplished in several different 

 regulators. 



FiG. 50. Beard's Large Regulator 



