84 OPTICAL PROJECTION 



The house-gas is connected as usual, and then the oxygen is 

 turned on very cautiously till there is enough supply. When- 

 ever the light goes down appreciably, a very little more is 

 turned on ; a bottle containing 6 cubic feet being sufficient, 

 with care, to last a single lantern nearly two hours. 



Without care, however, it will not do so ; and it will be 

 found particularly easy to use all the oxygen in less than one 

 hour. Care must be taken to use as little as is sufficient, and 

 to turn on as little more as possible at one time. It is only 

 further necessary in using oxygen thus, to be careful never to 

 turn off the oxygen at the jet. If this be done, the full pres- 

 sure of the cylinder rapidly accumulates behind the stop- cock, 

 and blows off the rubber tube. There is not the slightest 

 danger in this ; but it causes a serious waste of gas, which 

 may unpleasantly shorten the exhibition. 



Before compressed gas could come into at all general use 

 with all kinds of jets and lanterns, it was necessary to improve 

 the cylinders, to provide efficient regulators of the pressure, 

 and to reduce the price of the gas. 



44. The Cylinders. These are now made of a fine 

 quality of steel, less than one third of the weight and bulk of 

 the iron once formerly manufactured, much higher pressure 

 being employed. The common pressure for full cylinders is 

 1800 Ibs. to the square inch, but every cylinder is tested to 

 more than double its utmost real pressure before being used. 

 The following are fair average dimensions and weights as now 

 made. 



Capacity Size Weight 



6 ft. 12 x 31 in. 8 Ibs. 



12 18 x 3| 13 



20 32 x 4 21 



40 32 x 5| 38 



The worth of a cylinder depends entirely upon the 

 excellence of its valves, and a great many of those advertised 

 at unusually cheap rates are not to be depended upon, but 

 gradually allow the gas to escape. I have repeatedly known 



