144 Management of Light in Illumination. 



passing too freely out of it, by a retrograde motion, on 

 every puff of wind that may blow down into the top 

 of the glass chimney. 



In order more effectually to defend this lamp against 

 those descending blasts, and also from being blown 

 out by the air forced into the opening of the chimney 

 above, on lifting up the lamp very suddenly, the top of 

 the chimney is covered by a small conical roof, made 

 of thin sheet iron, two inches in diameter below and 

 about one inch and a quarter in height. This roof is 

 fixed in its place by means of three narrow vertical 

 slips of sheet iron, a quarter of an inch in width and 

 an inch and a half in length, which are riveted above 

 to the inside of the conical roof. These slips, which 

 are elastic, on being forced together, enter the glass 

 chimney, and by pressing against its sides keep the 

 roof fixed in its place. 



It might have been apprehended that this roof 

 would have so checked the ascending current of air in 

 the chimney as to diminish the rapidity of the com- 

 bustion and impair the brilliancy of the light ; but this 

 has not been found to be the case. The three slips of 

 sheet iron by which the roof is fixed in its place are 

 so arranged that the level of the lower part of the roof 

 is about one tenth of an inch higher than the extremity 

 of the glass chimney; and a greater height has not 

 been found to be necessary to give a free passage to 

 the air. 



These different contrivances defend the lamp so 

 effectually against both wind and rain, that the lamp 

 may without any risk be used in the open air instead 

 of a lantern, and even in stormy weather. 



The use of the roof is not absolutely necessary 



