Airships past and present, together with chapters on the use of balloons in connection with meteorology, photography and the carrier pigeon . equal to that of a column of mercury2992 in. high ; the specific gravity of mercury is 13*59, andtherefore the pressure of the air on a square inch is 14*706 us suppose the air to be contained in a cylinder, which isclosed by an airtight piston, the cross section of the cylinderbeing 1 square inch in area. Let us further suppose that thislittle piece of apparatus is placed beneath the receiver of the airpump. It will then be found that, if the pi
Airships past and present, together with chapters on the use of balloons in connection with meteorology, photography and the carrier pigeon . equal to that of a column of mercury2992 in. high ; the specific gravity of mercury is 13*59, andtherefore the pressure of the air on a square inch is 14*706 us suppose the air to be contained in a cylinder, which isclosed by an airtight piston, the cross section of the cylinderbeing 1 square inch in area. Let us further suppose that thislittle piece of apparatus is placed beneath the receiver of the airpump. It will then be found that, if the piston is to be kept inposition without allowing the gas in the cylinder to expand, itwill be necessary to load it with a weight of 14*7 lbs. If the pistonis loaded with a weight of 29*4 lbs., the volume of the gas will be THE THEORY OF THE BALLOON. 31 reduced by one half ; the pressure of the gas will therefore bedoubled, and its density similarly increased. Boyle and Mariottehave therefore stated that the volume of a gas is inverselyproportional to its pressure or density. It is now possible with the aid of Boyles law to calculate the. Fig. 16. -Stockholm, seen from a height of 1,600 feet. Photograph by OskarHalldin. lift which acts on a balloon at different heights, or withdifferent atmospheric pressures. Let us suppose that the baro-metric pressure is that of 30 in. of mercury, and that the liftis 1,600 lbs. If the pressure sinks to 29 in., the lift will becomefg of 1,600 lbs., , 1,550 lbs. The difference between thesetwo forces is 50 lbs., and corresponds to the weight of about twosacks of ballast. At a height of 6,600 ft. a cubic foot of air 32 AIESHIPS PAST AND PEE SENT. weighs only 0064 lb., and a cubic foot of hydrogen would weigh0*00396 lb. It is therefore possible in this way to determine thegreatest height to which it is possible to ascend, if the dead-weight of the balloon is already known. Hitherto we have assumed the temperature to be constant, andit is necessa
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