Airships past and present, together with chapters on the use of balloons in connection with meteorology, photography and the carrier pigeon . pressure causecorresponding changes in the volume. An increase of internalpressure can be relieved by an automatic valve, but a contractionis at once noticed by the creases on the envelope. Xo doubt anydecrease in volume can be met by pumping air into the balloon ;but this naturally dilutes the gas, besides gradually creating avery explosive mixture. The best plan would be to pass moregas into the balloon ; but owing to the weight of the cylindersused fo
Airships past and present, together with chapters on the use of balloons in connection with meteorology, photography and the carrier pigeon . pressure causecorresponding changes in the volume. An increase of internalpressure can be relieved by an automatic valve, but a contractionis at once noticed by the creases on the envelope. Xo doubt anydecrease in volume can be met by pumping air into the balloon ;but this naturally dilutes the gas, besides gradually creating avery explosive mixture. The best plan would be to pass moregas into the balloon ; but owing to the weight of the cylindersused for storage, it is impossible to take compressed gases on a DEVELOPMENT OF THE DIRIGIBLE BALLOON. 45 journey, though some method of storing gas in a liquid form mayin the future be available. The use of air-bags is therefore theonly solution ; when the volume of the envelope tends to increase,the air is pressed out of the receivers, and when it contracts airis sucked in. These air-bags can be mounted in the balloonin three different ways. According to the first method, theenvelope is made with two coverings over a portion of its Fig. 21.—Balloon, designed by General Meusnier. The two coverings lie tightly one upon the other when theballoon is full. But with a view to avoiding any unnecessaryloss of gas, it is better to fill the outer space with a certainamount of air at the start, so that the volume of enclosed aircorresponds to the increased bulk at the desired height. Thevalve will, therefore, only be opened when the balloon has risento the proper level. The most ordinary method consists insimply putting air-bags inside the balloon. Their size dependson the height to which it is intended to rise, seeing that thisdetermines the amount the balloon will expand. Such air-bags 46 AIKSHIPS PAST AND PRESENT. were used by the brothers Robert on the occasion of theirascent with the Duke of Chart res. The third method consists inhaving two separate envelopes, the inner one con
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