Locality.Dates.Differences.
June, 1853.
Burlington, Vt. 14thto30thranged from 8°to 17°
Montreal 14thto30th " 6 to 17
Poultney, Iowa 10thto30th " 9 to 16
Washington 20thto30th " 8.5 to 16
Baltimore 13thto30th " 7.4 to20.2
Savannah 13thto30th " 5.2 to17.3
Austin, Texas 10thto30th " 4 to 24
Clarkesville, Tenn. 4thto30th " 10.3 to20.5
August.
Bloomfield, N. J. 9thto14th"5 to15
Austin, Texas 6thto12th"0 to19
Philadelphia 10thto15th"8 to14
Jacksonville, Fla.10thto15th"6 to 8

Observations by Lieut. Gillis, at Washington, give mean differences between wet and dry thermometers, from March, 1841, to June, 1842, as follows:

Observations at 3 P. M.:

Jan.Feb.Mar.Apr.May.June.July.Aug.Sept.Oct.Nov.Dec.
3°.084°.406°.475°.377°.058°.038°.895°.295°.634°.614°.772°.03

A mean of observations for twenty-five years at the Radcliffe Observatory, Oxford, England, gives a difference between the wet and dry thermometer equal to about two-thirds the difference, as observed by Lieutenant Gillis, at Washington.

On the 12th day of August, 1853, in Austin, Texas, the air was perfectly saturated at a temperature of 76°, which was the dew-point, or point of the thermometer at which dew began to form. The dew-point varies according to the temperature and the humidity of the atmosphere; it is usually a few degrees lower than the temperature of evaporation—never higher.

From observations made at Girard College, by Prof. A. D. Bache, in the years 1840 to 1845, we find, that for April, 1844, the dew-point ranged from 4° to 16° lower than the temperature of the air; in May, from 4° to 14° lower; in June, from 6° to 20° lower; in July, from 4° to 17°; in August, from 6° to 15° lower; and in September, from 6° to 21° lower. The dew-point is, then, during the important months of vegetation, within about 20° of the temperature of the air. The temperature of the dew-point, as observed by Prof. Bache, was highest in August, 1843, being 66°, and lowest in January, 1844, being 18°; in July, 1844, it was 64°, and in February, 1845, it was 25°. Its hourly changes during each day are quite marked, and follow, with some degree of regularity, the changes in the temperature of the air; their greatest departure from each other being at the hottest hour of the day, which is two or three hours after noon, and the least at the coldest hour which is four or five hours after midnight. The average temperature of the dew-point in April, May, and June, 1844, was, at midnight, 50½°, air, 57°; five hours after midnight, dew-point, 49°, air 54°; three hours after noon, dew-point, 54°, air, 63½°. The average temperature for July, August and September, was, at midnight, dew-point, 58½°, air, 65°; five hours after midnight, dew-point, 58°, air, 62°; three hours after noon, dew-point, 60½°, air, 78°. The average temperature for the year was, at midnight, dew-point, 42°, air, 48°; five hours after midnight, dew-point, 41°, air, 46°; three hours after noon, dew-point, 44½°, air, 59°.

The relative humidity of the atmosphere, or the amount of vapor held in suspension in the air, in proportion to the amount which it might hold, was, in the year 1858, as given in the journal of the Franklin Institute, for

Philadelphia.Somerset Co.
April 49per cent.—2 P. M.
May 59 " 72 "
June 55 " 63 "
July 50 " 61 "
August 55 " 58 "
September50 " 57 "

The saturation often falls to 30 per cent., but with great variability. Evaporation goes on most rapidly when the per centage of saturation is lowest; and, as before observed, the cause of the excess of evaporation in this country over that of England is the excessive humidity of that climate and the dryness of this. It has also been said that there is greater need for drainage in the United States on this account; and, as the warmth induced by draining is somewhat, in its effect, a merchantable product, it may be well to consider it for a moment in that light.