Page:Popular Science Monthly Volume 51.djvu/324

312 would make it. The sketch presented herewith will give a fair idea of the difference between reality and the general ideal, the small car shown in solid lines being large enough to carry all the passengers or freight that the balloon could sustain, and the one in dotted lines about the size generally shown in illustrations of air ships. The sketch is not above criticism, since it does not give the location of the motor or any means for revolving the propeller, but that is a peculiarity of the majority of air-ship pictures, and the writer may be pardoned for following a common custom,



especially as the object of the sketch is only to show the relation between size and carrying capacity.

That this sketch is not exaggerated can be easily shown. The balloon is supposed to be one hundred and twenty feet long and twenty feet in diameter, the taper at each end being forty feet. From these dimensions it will be seen that the displacement is about twenty-one thousand feet, and the sustaining capacity about fifteen hundred pounds. Now, the first thing that any conservative engineer would admit would be that the apparatus could not be constructed within this weight if the same factor of safety were used as is customary in designing any ordinary structure; hence, if any carrying capacity is to be obtained, the weight and strength of every part must be reduced to a point not regarded as permissible in ordinary practice. Following this course, we can assume the weight of the whole ship at one thousand pounds, which would certainly be light considering its size; we would then have a net carrying capacity of five hundred pounds—equal to, say, four men. The car is drawn four foot square and six feet high, which is ample for four passengers. A contemplation of the difference between the size of the balloon and the car is enough to dampen the ardor of the most enthusiastic believer in the possibilities of aerial navigation.

It may be claimed that by the use of aëroplanes the size can