WITH reference to Mr. Ellis's letter in NATURE, vol. xx. p. 576, it may be enough to point out that, as Table 77 gives only the mean temperature of each day and month of the year for the whole period of the twenty years' observations, we must look elsewhere for the mean temperatures of the months of each successive year; and that this information is not furnished by Table 52, seeing that the means of that table have been prepared without correction for omitted days. Could Table 125 have been accepted as giving accurate mean temperatures this information would have been before us; but as matters stand a table showing the mean monthly and annual temperatures of Greenwich during each of these twenty years remains still to be constructed. An explanation as to how the daily mean values for those days on which no photographic value was available, were obtained in constructing Table 77, and a statement of the daily inequality of temperature of the underground apartment in which the photographic barometer is placed, would enable meteorologists to value even more exactly the highly important results of the Greenwich Meteorological Observations.