80 THE BOOK OF THE LANTERN. 



each lantern alternately has the great merit of saving 

 nearly half the gas bill. It will be noticed in the cut of 

 the dissolving tap that it is furnished with two small stop 



cocks, which are fixed on vertical tubes near its centre. 

 These tubes are by-passes which allow a small quantity 

 of gas to pass to each burner, although the lever has 

 shut off the main supply. The necessity for this arrange- 

 ment is obvious ; without it the lantern not in actual use 

 would be totally extinguished. In using the blow -through 

 form of jet :both by-passes, must be employed, but with 

 the ;mixed jet the hydrogen by-pass only is necessary. 

 The first thing to be done in operating with a double 

 lantern is to light the hydrogen in both lanterns, and to so 

 adjust the by-pass that it will admit just enough gas to 

 the lantern not in use to give a flame about half an inch 

 high. When the blow -through jet is used the correspond- 

 ing oxygen tap must be manipulated to furnish just enough 

 of that gas to tinge the hydrogen flame. If this precaution 



