. Memoirs of the American Academy of Arts and Sciences. Fic. 3. SouUTi Fig. 4. pritchards wedge photometer. 307 Here the probable error of the mean is ^\ of one division, or rather less than j\of one per cent of the whole. The probable error of a single is about1*0 of one division, or rather less than i of one per cent of the whole. Even inthis case, however, the probable error is compounded of that due to the instrumentand that due to the changing temperature of the source. If, however, we measure the solar heat, the result is less favorable; for, as I haveelsewhere remarked, the
. Memoirs of the American Academy of Arts and Sciences. Fic. 3. SouUTi Fig. 4. pritchards wedge photometer. 307 Here the probable error of the mean is ^\ of one division, or rather less than j\of one per cent of the whole. The probable error of a single is about1*0 of one division, or rather less than i of one per cent of the whole. Even inthis case, however, the probable error is compounded of that due to the instrumentand that due to the changing temperature of the source. If, however, we measure the solar heat, the result is less favorable; for, as I haveelsewhere remarked, there is always found an incessant Muctuation of the heat trans-mission from minute to minute under an optically clear sky, or, in other words, thebolometer constantly perceives haze and mist which the eye does not. The same day on which the above Leslie cube readings were taken, presentedto the eye a fair, blue sky, with some curaulous clouds. Except for the passageof a cloud between the ninth and tenth reading, in the following series of twentyreadings
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