= 0·08986 × (_H_/760) × 1/(1 + 0·00367_t_) gram. For aëronauts it is very useful to know, besides this, the weight of the air at different heights, and I therefore insert the adjoining table, constructed on the basis of Glaisher's data, for the temperature and moisture of the atmospheric strata in clear weather. All the figures are given in the metrical system--1,000 millimetres = 39·37 inches, 1,000 kilograms = 2204·3375 lbs., 1,000 cubic metres = 35,316·6 cubic feet. The starting temperature at the earth's surface is taken as = 15° C., its moisture 60 p.c., pressure 760 millimetres. The pressures are taken as indicated by an _aneroid barometer_, assumed to be corrected at the sea level and at lat. 45° C. If the height above the level of the sea equal _z_ kilometres, then the weight of 1 cubic metre of air may be approximately taken as 1·222- 0·12_z_ + 0·00377_z_^2 kilogram. +--------+-----------+--------+--------+--------------------+ |Pressure|Temperature|Moisture| Height | Weight of the air | | | | |(metres)|(1,000 cubic metres)| |--------+-----------+--------+--------+--------------------+ | 760 mm.| 15° C. | 60 p.c.| 0 | 1222 kilos. | | 700 " | 11·0° " | 64 " | 690 | 1141 " | | 650 " | 7·6° " | 64 " | 1300 | 1073 " | | 600 " | 4·3° " | 63 " | 1960 | 1003 " | | 550 " | -1·0° " | 62 " | 2660 | 931 " | | 500 " | -2·4° " | 58 " | 3420 | 857 " | | 450 " | -5·8° " | 52 " | 4250 | 781 " | | 400 " | -9·1° " | 44 " | 5170 | 703 " | | 350 " | -12·5° " | 36 " | 6190 | 624 " | | 300 " | -15·9° " | 27 " | 7360 | 542 " | | 250 " | -19·2° " | 18 " | 8720 | 457 " | +--------+-----------+--------+--------+--------------------+ Although the figures in this table are calculated with every possible care from average data, yet they can only be taken approximately, for in every separate case the conditions, both at the earth's surface and in the atmosphere, will differ from those here taken. In calculating the height to which a balloon can ascend, it is evident that the density of gas in relation to air must be known. This density for ordinary coal gas is from 0·6 to 0·35, and for hydrogen with its ordinary contents of moisture and air from 0·1 to 0·15. Hence, for instance, it may be calculated that a balloon of 1,000 cubic metres capacity filled with pure hydrogen, and weighing (the envelope, tackle, people, and ballast) 727 kilograms, will only ascend to a height of about 4,250 metres.