Page:Memoir of a tour to northern Mexico.djvu/116

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Are prepared from my meteorological journal, kept on the road. Some of the columns may require an explanation.

The column "boiling point of water," refers to my observations with two thermometers, constructed by my order, by J. W. Edmonds, of Boston, each varying from 85 to 100° Celsius, and every degree divided in tenths. I made many experiments with them on the road, to find the relative difference between the boiling point of water and the mercurial column of my barometer. From about 50 such observations, made within the range of from 23 to 29 inches of the barometer, I abstracted the general result, that 1 inch of my barometer at the temperature of 32° Fahr., was = 1°.04138 boiling point Therm. Cels., and 1° Th. C. = 0″.96026 of the barometer. But, at the same time, I have come to the conclusion, as others before me, that the determination by the boiling point of water can never in correctness equal the barometrical measurement. A difference in fuel, in water, in the size of the vessel, in draught of air, &c., is apt to produce such a discrepancy in the relative boiling point, that this method will answer well enough as a correlative proof of the barometer, and for heights, where several hundred feet, more or less, is not a matter of consideration, but that it will never be capable of supplying the place of the barometer.

The dew point, found by excess of temperature of the dry over the wet bulb, is calculated according to "tables for the determination of the dew-point," given in the Encyclopœdia BritanicaEncyclopædia Britannica [sic], and republished in the "Report to the Navy Department of the United States on American coals, by Professor Walter R. Johnson: Washington, 1844." Observations beyond the reach of these tables, I calculated according to the rule given by Professor Espy: "The dew-point, when it is not very low, may be nearly obtained by multiplying the difference between dry and wet bulb temperature with 103, dividing the result by the wet-bulb temperature, and subtracting the quotient from the dry-bulb temperature; the remainder will be the dew point."

In the column "wind" the force of the wind is designated, as recommended by Professor Espy, by numbers from to 6; being a calm, 1 a very gentle breeze, 2 a gentle breeze, 3 a fresh wind, 4 a strong wind, 5 a storm, and 6 a hurricane.

The clearness of the sky is also marked in numbers from to 10; representing entire cloudiness, and 10 entire clearness.