SIGNALLING 



183 



This form of lantern may be used with a screen similar to the heliograph screen, 

 but to do so the light must burn continuously during signalling. This more than 

 doubles the consumption of carbide, since one charge of the generator will furnish 

 gas for only 2 hours' continuous light. The speed of operation with a screen is eight 

 to fourteen words per minute. 



Fig. 106 Acetylene signal-lantern with tripod 



As usually employed, the screen is dispensed with and a special form of key 

 is used which controls the supply of gas to the burners. A small amount of gas 

 is admitted continuously. Depressing the key causes a full flow of gas to be admitted 

 to the burner, giving a bright flash lasting as long as the key is held down. The 

 response to the key is instantaneous. Operated in this manner, less carbide is used 

 but the speed is slightly reduced, being about five to ten words per minute. Smoke, 

 fog, rain, and also bright moonlight are the principal impediments to the use of this 

 lantern. The range under ordinary conditions of operation does not exceed 30 miles 

 when viewed with a telescope, and about half this when observed with the naked eye. 



(ft) Electric Lamps. For comparatively short-distance work the electric signal- 

 lamps, such as are used by the British Army, are very convenient. Such a lamp is 

 the Stevens-Lyon lamp, manufactured in London. This lamp weighs 10* pounds 



