Airships past and present, together with chapters on the use of balloons in connection with meteorology, photography and the carrier pigeon . n aspirator-psychrometer, which forms an indispensableitem in the outfit of the scientific balloonist. This instrument contains two thermometers, having theirmercury bulbs protected by highly polished metal tubes, abouthalf an inch in diameter. These tubes are open at the top, andcommunicate at the bottom with a central metallic tube, aboutone inch in diameter and 8 inches long. At the top of the instru-ment there is placed a clockwork apparatus, driven
Airships past and present, together with chapters on the use of balloons in connection with meteorology, photography and the carrier pigeon . n aspirator-psychrometer, which forms an indispensableitem in the outfit of the scientific balloonist. This instrument contains two thermometers, having theirmercury bulbs protected by highly polished metal tubes, abouthalf an inch in diameter. These tubes are open at the top, andcommunicate at the bottom with a central metallic tube, aboutone inch in diameter and 8 inches long. At the top of the instru-ment there is placed a clockwork apparatus, driven by a spring,which serves to put two metal discs in rotation. The rotation ofthese discs sucks the air through the central tube, and con-sequently past the thermometer bulbs, at a speed of 8 or 10 ft. per 1 See Assmanns Wissenschaftliche Luftfahrten, vol. i., page 56. SCIENTIFIC BALLOONING. 247 second. The rays of the sun are reflected by the polished metalsurrounding the bulbs of the thermometers, which are thereforeprotected from external influences and register the temperatureof the air as it is sucked past them. In this way the true. Fig-. 150.—Basket fitted with instruments according to the method proposed by Assmann. temperature of the atmosphere can be found, supposing that thethermometers are kept at a sufficient distance from the observers,etc., to be free from any of the effects of radiation that may bedue to the contents of the basket. The instrument is preferablymounted on [some kind of support which keeps it at a suitabledistance from the basket on the outside. It is then quickly 248 AIRSHIPS PAST AND PRESENT. drawn up to the edge of the basket for the purpose of takinga reading; or, if very great accuracy is needed, it may be readthrough a telescope. The working of this instrument has beentested by long exposure to the rays of the sun on the top of theSantis, and its readings were found to be very accurate. Professor Assmann proposed to Professors Berson and Sirr
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