METEOROLOGY 435 depth of 3 ft., while annual variations are barely observable at a depth of 80 ft. The effect of the sun's heat upon the water of the ocean differs in some important respects from its effect on the continents : first, in that a large percentage of heat is rendered latent in evaporating the surface water of the ocean ; secondly, in that the specific heat is much larger for water than for dry earth ; and finally, in that the mobility of the water permits of a very extensive sys- tem of convection. According to the observa- tions of Lenz (1829), Carpenter (1870), Thom- son (1874), &c., the average temperature of the ocean at depths greater than 600 ft. is that of the maximum density of water at the pres- sure to which it is subjected. The tempera- ture of a layer a few inches in thickness on the immediate surface of the water can, when the ocean is very still, be raised as high as 90 F. ; but in the general disturbed condition of the water its surface temperature is much be- low that of the adjacent stratum of air. As a secondary effect of the influence of the ocean, must be noted the fact that the heat rendered latent in the evaporation of its surface water in great part returns to the atmosphere when that vapor is condensed to cloud and rain. Having thus indicated the original sources of atmospheric temperature, we come to the con- sideration of that subject itself. In order that the temperatures measured in different portions of the world may be comparable among them- selves, it is necessary that uniformity should be secured in the exposure of the thermome- ter to such influences as can affect its indica- tions. As a general rule, in ascertaining the temperature of the lowest stratum of air, the thermometer should be elevated not less than 5 nor more than 50 ft. above the earth's sur- face, and should be surrounded on all sides, at a distance of from 1 to 5 ft., by a light double latticework, or equivalent structure, which can itself rapidly follow the varying temperature of the air, and prevent all radiation of heat except that which takes place between the thermometer and the interior of the lattice- work. But such a thermometer cage is scarce- ly practicable in the investigations of the tem- perature of the upper strata, so far as they can be reached by aeronauts, and numerous but unsatisfactory studies have been made into the relations between the indications of protected and unprotected thermometers. Some knowl- edge as to the temperature of the upper strata is given by the study of the refraction by the atmosphere of the rays of light which reach us from the celestial bodies. The most important features of atmospheric temperature are its va- riation with the altitude, its diurnal variation, its annual variation, and its geographical dis- tribution. On the three latter points the mass of information can best be presented by graphic methods, as in the accompanying diagrams. But the irregularities of the temperatures at various altitudes may be better seen by the study of the following table : Diminution of Temperature with increasing Altitude, as shown by Observations in Balloons. 1 < Feet. 0,000 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000 11,000 12,000 13,000 14,000 15,000 16,000 17,000 18,000 19,000 20,000 21,000 22,000 23,000 24,000 25,000 26,000 Gay-Lussac aiid Blot. 1 fl GlaUher and Coxwell. 1804, Sept. 16 1852, Nov. 10 1863, July 11. 1862, Sept. 5. 18 Apr Aec. 61-0 59-2 57-0 51-0 48-2 44-2 40-1 85-9 33, 1 18. Desc. 57 -O" 87-4 50-0 46-2 -o* Desc. Asc. 59-5 56-0 Desc. '54 : 6 ; 53-5 '49 : 2' 45-5 41-0 40-0 42-4 88-7 36-5 85-8 45-0 45-0 41-0 36-5 40-0
34-9 84-0 82-4 31-1 28-6 25-5 22-1 17-8 15-8 14-2 12-9 9-7 5-4 2-8 1-8 6-2 9'2 10-8 85-8 82-5 '54 : 4' 52-0 47-5 50-1 48-8 47-6 48-8 36-8 86-9 30-9 29-2 23-8 20-9 14-5 26-0 '26 : 6' 29-0 37-4 84 : 2 32-6 81-2 '26 : 6' 81-5 81-0 26-5 24-5 23-0 'is'6' 82-0 'si : 6 23-0 21-0 20-0 17-5 12-0 25-0 21-0 'i6 : 5' 17-0 16-0 83-0 31-0 83-0 '35 : 6' 81-5 24-5 19-2 29 : 7 18-0 16-5 28 : 6 15-0 15-0 8-0 11-0 4-5 'ii : 6' 'ii : 6' 2-0 2 : 6' 'l2 : 6' 10-5 13-0 16-5 12-5 12-0 'l4 : 5' 'i2 : 5' 12-0 16-5 16-0 16-0 27-0 19-0 o-o 2-0 5-0 37,000 11-9 11-9 The decrease of temperature with increase of elevation has apparently a diurnal and an an- nual change, and 30 ascensions in 1868 showed that it may very probably often during the night be reversed into an increase instead of decrease, at least for the lowest 2,000 ft. Above the clouds the temperature decreases very steadily. As an average of all of his midday ascensions, Glaisher says that the diminution in the first 1,000 ft. was 4'5 with a cloudy sky, and 6-2 with a clear sky. In an ascent made on April 6, 1864, remarkable vari- ations were met with, such that at 10,000 ft. the thermometer registered the same as on the earth's surface; but on this occasion clouds and fogs alternated up to the highest point at- tained by the balloon. Diagram No. I. shows the diurnal variations of temperature for Jan- uary and July, as deduced from observations continued through several years at stations in different geographical positions. In this table the average daily temperature of the month is assumed as the zero line. The annual vari- ations for a few representative stations are shown graphically in diagram No. II., where even the minuter features of the yearly changes are developed by the use of Dove's five-day means. It will be noted that the maximum temperature occurs at Madras at the time of the solstice, while at stations further north it occurs two to five weeks later. Diagrams No. III. and IY. give the average distribution of mean temperatures over the surface of the earth as shown by means of the isothermal lines for January and July respectively, as published
