METEOROLOGY. 309 



heat of 1 per cent, corresponds approximately to a change of ground 

 temperature of 1° C. The variations above found by Frolich would, 

 therefore, if they endured for any length of time, produce a change in 

 the earth's temperature of six or eight degrees. 



181. [It is certainly very desirable that Frolich's observations should 

 be maintained continuously at several stations on the earth's surface 

 where atmos})heric changes are comparatively rare, as it is scarcely pos- 

 sible to think that we shall ever master the peculiarities of meteorology 

 until we thoroughly understand the variations in the prime factor of 

 all— the solar heat.] {Z. 0. G. M., xix, p. 209.) 



182. Dr. H. 0. Vogel, of Potsdam, comments upon these results of 

 Frolich's and maintains that the observations do not distinctly show 

 changes in the solar heat that are appreciable in comparison with the 

 uncertainty of each day's observations ; that possibly in time one may 

 accumulate sufficient observations by Dr. Frolich's method to detect the 

 short variations, but that five days of observations are not sufficient. 

 With reference to Frolich's opinion that variations in the solar heat, if 

 any exist, have some connection with the size and nnmber of the sun's 

 spots, Yogel states that accurate measures of the sun-spot area in April, 

 1882, on a day that was specially rich in spots, showed that the solar 

 heat could only have been diminished by about four- tenths of 1 per cent. 

 The observations made^t Potsdam on the very days of Dr. Frolich's 

 observations show that the influence of the few spots then visible uuist 

 have been extraordinarily- small, and that his temi)erature observations 

 must have at least ten times greater accuracy than now before he can 

 detect such influence. {Z. 0. G. M., xix, p. 259.) 



183. Dr. O. Frolich, in some remarks on methods of measuring solar 

 heat, states that he cannot agree with Langley in the assumption that 

 tiie atmospheric coefficient of absorption is proportional to the barome- 

 tric pressure, nor with his conclusions that the logarithmic law holds 

 only for homogeneous rays and not for the whole spectrum. Frolich's 

 investigation of the absorption due to the altitude of the sun has the 

 advantage of Langiey's, in that the latter uses only two observations at 

 .difl'erent altitudes while the former uses from four to lifteen. Frolich 

 also doubts the accuracy' of Langiey's reduction to a standard measure. 

 He proposes to explain the movements of the granite pillar observed 

 by Fiirster (see chapter xii) as due, not to a change in the amount of 

 solar neat, but to a change iu its character such that at the time of the 

 solar spot frequency the earth or the pillar absorbs relatively more 

 or less heat. Frolich defends himself against the criticisms of H. C. 

 Yogel to the effect that he has not been fully on his guard against 

 constant errors by referring to the fact that the inost important novel 

 feature of his work lay iu the constant reference to a normal standard, 

 and that nothing can diminish the certainty of the conclusion that his 

 observations demonstrate a sensible variation of solar heat during Au- 

 gust. In reference to the possibility of detecting changes in the sun's 



