Page:Popular Science Monthly Volume 1.djvu/687

671 locality at which the observation is made. The former represents the quantity of heat over a long line, including many localities. It is, therefore, the better form, and furnishes more trustworthy results.

Turning now to the records of the city of New York, as obtained from the sources above specified, we find they are continuous from 1821 to the present time. It would extend this report unduly were we to enter on an examination of each of these years respectively. Making a selection, then, let us compare the following groups of five years—first, from 1821 to 1827; second, from 1831 to 1837; third, from 1841 to 1847; fourth, from 1866 to 1872. It will be understood that the months selected are January, February, and March.

The mean for January, February, and March, for thirty-three years, is 32.90 degrees. The mean for the above selection is 33.06 degrees.

The evidence thus derived from thermometric observations corroborates that derived from the freezing of the river, and undeniably leads to the conclusion that, if there has been any change in the winter climate of New York, it cannot be demonstrated by the extant thermometrical records of the last fifty years. This, therefore, adds weight to Humboldt's conclusion that there has been no sensible change in the Atlantic States since the time of their first settlement.

Let us next see what is the evidence afforded by an examination of the Philadelphia records. As in the preceding case, a discussion of all these would be too lengthy. They go as far back as 1748, but present, however, a broken series. Selecting from this, here and there, periods of five years, we may thus group them: first, from 1766 to 1772; second, from 1797 to 1803; third, from 1821 to 1827; fourth, from 1831 to 1837; fifth, from 1851 to 1857.