*Methods of Inflation.* The average capacity of the kite balloons used for observation purposes is 28,000 cubic feet. They are inflated with hydrogen either from a portable generating plant forming part of the equipment of the balloon company or from a supply carried under high pressure in heavy steel "bottles" similar to those used for transporting oxygen or carbonic acid gas intended for industrial use. Since the balloon companies are stationed about 4 miles back of the firing line, the use of the portable plant is practical, but it has been found more economical and more convenient to generate the gas on a large scale at special establishments in France and England and send it to the front in containers. With a portable plant, several hours are necessary to inflate the gas bag, whereas with a large supply of the gas at hand under high pressure, the operation may be carried out in less than an hour. The balloon naturally works under the same difficulties as all lighter-than-air craft, that is, there is a constant leakage of the hydrogen through the fabric in addition to that lost by the expansion of the gas on warm days when the summer sun beats down directly on the gas bag. Where a field generating plant is employed, quick inflation of a new balloon or replacement of loss is accomplished by the used of several "nurses", Fig. 27. These are simply large gas bags which are kept replenished by the gas plant working constantly, in other words, they are storage tanks, and when it is necessary to inflate the balloon quickly, their contents are simply transferred to it. [Illustration: Fig. 27. Landing Big Kite Balloon at Training Station "Somewhere in England." "Nurse" in Background _Copyright by Underwood and Underwood, New York_]