64 THE BOOK OF THE LANTERN. 



quired to be shown. Using a disc of from 15 to 18 feet in 

 diameter the exhibitor will do well to commence when the 

 bags are full with two half -hundred weights on his pressure 

 boards. But when the gas has been so much used that the 

 upper board gets nearly horizontal the pressure will be 

 lessened, and the light will suffer to some extent. When 

 this happens, the experienced operator will place another 

 half-hundredweight in position, and the increased bright- 

 ness of the picture will quickly show the advantage of so 

 doing. 



When the gas or gases are drawn direct from steel 

 cylinders, or bottles, if a double or triple lantern is in 

 use, some form of regulator must be employed. The first 

 introduced, and perhaps the most perfect, is that patented 

 by Messrs. Oakley & Beard. 



Before proceeding to describe this important new de- 

 parture in lantern working, it may be as well to point out 

 one or two difficulties which are incidental to the ordinary 

 method of storing the gases required in india-rubber bags. 

 So far as the writer knows, one of these difficulties has 

 never been recognised in print. This difficulty is com- 

 prised in the fact that any kind of gas, if kept in an india- 

 rubber bag, quickly deteriorates. By the phenomenon 

 known by the term endosmose, two gases separated by a 

 porous diaphragm will effect a mutual exchange. Take 

 the case of an ordinary india-rubber tube used. for a table 

 lamp or gas stove. How quickly it begins to smell. 

 This is nothing else but the gas escaping through the 

 india-rubber, and carrying on an exchange with that other 

 gas the air which is outside. It would be interesting 



