Airships past and present, together with chapters on the use of balloons in connection with meteorology, photography and the carrier pigeon . urrounding atmosphere, except by night, when the temperaturesof the two are nearly the same, the gas being sometimes slightlythe colder of the two, owing to losses by radiation. It is very necessary to take good maps of the district. But on INSTRUMENTS. 193 a long journey, they are apt to be so numerous that they are nowoften replaced by maps on a very small scale, which are readby means of a magnifying glass. As this system is possibly amatter of some g
Airships past and present, together with chapters on the use of balloons in connection with meteorology, photography and the carrier pigeon . urrounding atmosphere, except by night, when the temperaturesof the two are nearly the same, the gas being sometimes slightlythe colder of the two, owing to losses by radiation. It is very necessary to take good maps of the district. But on INSTRUMENTS. 193 a long journey, they are apt to be so numerous that they are nowoften replaced by maps on a very small scale, which are readby means of a magnifying glass. As this system is possibly amatter of some general interest for other purposes than balloon-ing, it may be as well to describe it a little more fully. Themethod is due to an officer of the Bavarian Balloon Corps, namedvon Weinbach, who communicated his ideas to Dr Vollbehrof Halensee. An instrument, called the microphotoscope, wastherefore designed. It consists of two parts, which are quiteseparate from one another, viz., the eyepiece or magnifier, whichis used in daylight, and a lighting device, which is used by magnifier consists of a. lens, which is so mounted as to. Fig. 119.—Barograph, or recording barometer. be capable of moving in slots, either up and down, or to the rightand left. Microphotographs, which represent photographicreductions of maps published on a larger scale, are taken oncelluloid films, and mounted in position between thin sheets ofglass, two inches square. The lighting arrangement contains asmall electric glow-lamp and a battery, the lamp being switchedon and off as required. This arrangement works well on nightjourneys, and it is generally possible to determine the localityby noting the lights in the towns and the positions of therailways. The daylight apparatus weighs 4 oz., the lighting-apparatus 5 oz., and the complete thing together with the caseweighs 13 oz. The price is twenty-five shillings, which mayeasily be saved in the cost of maps. It is extremely necessary to see that the
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