Airships past and present, together with chapters on the use of balloons in connection with meteorology, photography and the carrier pigeon . pe of the balloon, and of the there is a limit to human effort, and since the resistance ofthe air increases with the square of the velocity, it soon becomesevident that even in gentle breezes the only method of over-coming the resistance would be by means of propellers, drivenat high speeds. The effect produced by rudders is similar tothat produced by them on ships, always supposing the balloon isunder weigh. The proposals to use vertical sails
Airships past and present, together with chapters on the use of balloons in connection with meteorology, photography and the carrier pigeon . pe of the balloon, and of the there is a limit to human effort, and since the resistance ofthe air increases with the square of the velocity, it soon becomesevident that even in gentle breezes the only method of over-coming the resistance would be by means of propellers, drivenat high speeds. The effect produced by rudders is similar tothat produced by them on ships, always supposing the balloon isunder weigh. The proposals to use vertical sails betray acomplete misconception of the laws underlying the movementsof balloons. If a balloon, filled with gas, floats in the air, all itsparts will move with the breeze, and at the same speed. A sailwould therefore hang just as limply as it would do in a completecalm. It would be different if it were possible to give the balloona smaller or greater velocity than the wind, and in such a casea pressure would be exerted on the sail. The explorer Andreeproposed to work on this idea in the simplest fashion. He 40 AIRSHIPS PAST AND Fig-. 19.—Balloon with sail, and with guide-rope fastened to the rinj intended, with the help of the friction caused by a number ofropes dragged along the ground, to cause the balloon to go ratherslower than the wind. A sail was then to be hung out, and DEVELOPMENT OF THE DIRIGIBLE BALLOON. 41 placed in such a position that the force of the wind acting on itwould drive the balloon in any desired direction. Tests haveshown that with clever management it is possible to produce inthis way a slight deviation from the direction of the wind. It isalso known that surfaces slightly inclined to the horizontal willproduce a slight movement of the balloon as it rises and Montgolfier knew this and tried to utilise the idea inone of his models. Since his time many others have alsoworked on the same lines, but no practical success has beenachi
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