SHADE TREES. 



227 



often detect the odor of gas in the cellar, as the gas will follow the ex- 

 terior of the pipe leading into the cellar, and often escapes into sewers, 

 underground conduits, hydrants, etc. Wells are often badly contami- 

 nated even when located some distance away, and in the winter gas 

 leakage becomes a source of danger to near-by greenhouses. 



There is considerable difference in the resistance of soils to gas. In 

 gravelly soils we have known gas to travel 2,000 feet when the ground was 

 frozen and escape into the cellar of a house, while in heavier soils it is 

 more likely to be restricted to smaller areas. It requires a considerable 

 amount of gas to kill a large tree, although not so much as it would were 



Fig. 97. — Protective arrangement against injury resulting from leakage of illuminating 

 gas. A, cross section of protective conductor; B, adjustment of same to pipe; C, 

 black lines showing method of ai-rangement of protective device on street and house 

 service; D and E, vents for leakage. 



it not confuied so closely by the soil covers, especially in winter, and by 

 the impenetrable macadam and other styles of modern roadbeds. 



The danger to human life from illuminating gas is too great to be ig- 

 nored, and even with the present defective systems of distribution it is 

 not only possible but practicable largely to eliminate the dangers from 

 this source to trees as well as to human life by the use of certain devices 

 to prevent the leakage of gas into the soil. 



The comparatively recent introduction of joints welded by the acety- 

 lene flame may prove superior to the threaded or calked cement and 

 leaded joints in preventing leakage, but this system of laying street mains 

 has not been thoroughly tested out in cold climates. There are also pro- 

 tective arrangements covering the pipes designed to prevent leakage of 



