To get over this difficulty the Germans fitted their cylinders with internal siphon tubes so that actually liquid chlorine was forced into the air, where it evaporated without affecting the gas remaining in the cylinder. By this means the whole of the gas in the cylinder, amounting to forty-five pounds, is emptied into the atmosphere in less than three minutes. The sudden expansion of the chlorine in the air also makes both it and the surrounding air cold and helps to keep the cloud close to the ground. The actual handing of the gas attacks was allotted to two regiments of pioneers--the 35th and 36th Pioneer Regiments, which were specially organised for this purpose. These regiments have the ordinary organisation of two battalions per regiment, with three companies and a park or transport company per battalion. The rank and file are ordinary pioneers, but the officers are specially picked and include chemists, mechanical experts, meteorologists, and other men with special scientific qualifications. The choice of country in which to make a gas attack was a serious matter to the enemy. The gas of course will go with the wind, but it depends largely on what the country is like where the wind will go. The Germans themselves say they prefer a flat country without any marked under features and sloping gently toward our lines, just as they had at Ypres. Indeed, they went the length of saying that a gas attack could not be carried out in hilly or very broken country; and they suffered in consequence later on, through being taken unawares by the French in just such country in the Vosges when retaliation was commenced. But taking it altogether the Germans were wise in their choice of position. Another thing that had to be considered was the outline of their own trench system, so that they would not let off the gas in such parts of the line that it would float back and gas their own troops in the neighbouring trenches. To do this they invented a “factor of safety,” which represented an angle between the direction of the wind and the line of the trenches. No attack was to be made if the wind direction came within forty degrees of any trench within gassing distance. This worked very well. Another consideration was the strength of the wind. The wind must not be too strong when the gusts disperse the gas cloud, or it will weaken it so that it loses a lot of its effect and will be blown over the enemy too quickly; nor must it be so weak that it will take a long time to reach the opposing trenches.