auld: methods of gas warfare 47 



A wind of more than 12 miles an hour disperses the gas cloud 

 very rapidl}-. .\n upward current of air is the worst foe of gas. 

 The weight of the gas is not an important factor in carrying it 

 along, for it mixes rapidly with air to form the moving ■"cloud." 

 The time occupied by a gas attack is too short to permit of 

 much diffusion of the gas out of the original mixture. 



The gas attack must be planned very carefull}-. If the trench 

 line is very irregulai- it is likel}^ that gas will flow into a portion 

 of one's own trenches. The limits of safety in wind direction 

 are thus determined by the direction of the lines of the trenches. 

 The Germans use a 40° angle of safety; that means that on a 

 given straight portion of the front the wdnd direction must lie 

 between the two directions which make angles of 40° with the 

 neighboring sections of the front. The most suitable type of 

 country is where the ground slopes gently away from where the 

 gas is being discharged. The Germans made one mistake in 

 believing that hilly or wooded country would not do. This 

 was refuted by the French, who made a successful gas attack 

 in hilly and wooded country in the Vosges, as admitted in a 

 captured German report. If the country is flat like that about 

 Ypres, and the wind direction is right, there is very little diffi- 

 culty about making an attack, especially if the enemy does not 

 know anything about it. The element of surprise is important. 



German gas attacks are made by two Regiments of Pioneers, 

 with highl}^ technical officers, including engineers, meteorolo- 

 gists, and chemists. They brought their first cylinders into the 

 hne without our knowing anything about it, except from the 

 deserter's report which was not believed. The element of sur- 

 prise was greatly lessened when we began to know what to look 

 for and to recognize the sounds incident to the preparation of a 

 gas attack. 



The first attack was made with chlorine. If a gas attack is 

 to be made with gas clouds, the number of gases available is 

 limited. The gas must be easily compressible, easily made in 

 large quantities, and should be considerably heavier than air. 

 If to this is added the necessity of its being very toxic and of 

 low chemical reactivity, the choice is practically reduced to 

 two gases: chlorine and phosgene. Chlorine is to gas warfare 



