Airships past and present, together with chapters on the use of balloons in connection with meteorology, photography and the carrier pigeon . f a balloon, fitted with a parachute, at Lindenberg. very light construction ; it has merely to resist the internalpressure and carry the basket containing the has designed an arrangement whereby an alarumclock opens the valve after a certain time, and thereforecauses the balloon to descend after it has reached a certainheight. In order to prevent the effects of solar radiation, theballoon must be prevented from hovering in one positi
Airships past and present, together with chapters on the use of balloons in connection with meteorology, photography and the carrier pigeon . f a balloon, fitted with a parachute, at Lindenberg. very light construction ; it has merely to resist the internalpressure and carry the basket containing the has designed an arrangement whereby an alarumclock opens the valve after a certain time, and thereforecauses the balloon to descend after it has reached a certainheight. In order to prevent the effects of solar radiation, theballoon must be prevented from hovering in one position, so thatthe thermometers are continually brought into contact with s 258 AIKSHIPS PAST AND PRESENT. volumes of air, in a manner similar to that adopted in theaspirator-psychrometer. If the balloon is made to rise quicklyand then to fall at once, the thermometers give correct values;but if it is allowed to drift gently along, exposed to the rays ofthe sun, the readings will be too high. Attempts have beenmade to shield the instruments by placing them in a wickerbasket, covered with highly polished silver or nickel paper, but. Fig. 161.—Ascent of a box-kite containing meteorological instruments(Photograph by the Berliner Illustrationsgesellschaft.) this is not sufficient. Kites have been sent up in the earlymorning before sunrise, and figures, obtained in this way, havebeen compared with those recorded in bright sunlight. But thedaylight ascents are much the more important, as the effect ofthe sun on the atmosphere must on no account be has also invented a system by which rubber balloonswith diameters of one or two yards are sent up, graduallyexpanding as they rise, till they finally burst; a linen cap actsas a parachute, and the case with the instruments falls gently tothe ground. Such balloons will remain in the air from one to SCIENTIFIC BALLOONING. 259 three hours, and give good results. They are now employed inmost observatories. It is of cour
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