66 CONTEMPORARY SCIENCE 



to depend upon the hissing noises made by the escaping 

 gas, and upon the sense of smell. 



Another element of surprise was the sending out of 

 more than one cloud in an attack. After the first cloud 

 the men would think it was all over, but ten minutes or 

 half an hour later there would come another cloud on 

 exactly the same front. These tactics were very success- 

 ful in at least one case, namely, the attack near Hulluch 

 in 1916. Some of the troops discarded their helmets 

 after the first wave and were caught on the second, which 

 was very much stronger than the first. 



Efforts were also made to effect surprise by silencing 

 the gas. But silencers reduced the rate of escape so 

 greatly that the loss of efficiency from low concentration 

 more than made up for the gain in suddenness. Another 

 method was to mix the gas up with smoke, or to alternate 

 gas and smoke, so that it would be difficult to tell where 

 the gas began and the smoke ended. 



The last attack made on the British by this means was 

 in August, 1916. Since that time the Germans have used 

 gas three times on the West Front against the French, 

 and have also used it against the Italians and the Rus- 

 sians. It has been practically given up against the British, 

 although the method is by no means dead. 



The last attack was a slight set-back in the progress of 

 gas defense. The casualties had been brought down to a 

 minimum, and, as shown by the fact that the percentage 

 of deaths was high, protection was complete in all cases 

 where used, casualties being due to unpreparedness in 

 some form. The attack in question was brought on under 

 difficult conditions for the defenders, as it was made on 

 new troops during a relief when twice as many men were 

 in the trenches as normally. Furthermore, they had to 

 wear helmets while carrying their complete outfit for the 

 relief. This was the second time the Germans caught us 

 in a relief, whether through information or luck we can- 

 not say. 



