The temperature of evaporation is the reading of a wet-bulb-thermometer (the bulb being covered with moistened gauze) exposed to the natural evaporation; and the difference between that reading and the reading of a dry-thermometer, is the expression of the cold resulting from evaporation. When the air is nearly saturated, the temperature of the air rarely goes above 74 deg.; but, if so, the moisture in the air prevents the passing away of insensible perspiration, and the joint action of heat and humidity exhausts the vital powers, causing sun-stroke, as it is called. At New York city, August 12th to 14th, 1853, the wet-thermometer stood at 80 deg. to 84 deg.; the air, at 90 deg. to 94 deg.. The mortality, from this joint effect, was very great--over two hundred persons losing their lives in the two days, in that city. From very careful observations, made by Lorin Blodget, in 1853, at Washington, it was found that the difference between the wet and dry thermometer was 18-1/2 deg. at 4 P. M., June 30th, and 16 deg. at 2 P. M. on July 1st--the temperature of the air being 98 deg. on the first day, and 95 deg. on the second; but such excesses are unusual. The following table has been compiled from Mr. Blodget's notice of the peculiarities of the Summer of 1853: The dates are such as were selected to illustrate the extreme temperatures of the month, and the degrees represent the differences between the wet and dry thermometer. The observations were made at 3 P. M.: _Locality._ _Dates._ _Differences._ JUNE, 1853. Burlington, Vt. 14th to 30th ranged from 8 deg. to 17 deg. Montreal 14th to 30th " 6 to 17 Poultney, Iowa 10th to 30th " 9 to 16 Washington 20th to 30th " 8.5 to 16 Baltimore 13th to 30th " 7.4 to 20.2 Savannah 13th to 30th " 5.2 to 17.3 Austin, Texas 10th to 30th " 4 to 24 Clarkesville, Tenn. 4th to 30th " 10.3 to 20.5 AUGUST. Bloomfield, N. J. 9th to 14th " 5 to 15 Austin, Texas 6th to 12th " 0 to 19 Philadelphia 10th to 15th " 8 to 14 Jacksonville, Fla. 10th to 15th " 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._ 3 deg..08 4 deg..40 6 deg..47 5 deg..37 7 deg..05 8 deg..03 _July._ _Aug._ _Sept._ _Oct._ _Nov._ _Dec._ 8 deg..89 5 deg..29 5 deg..63 4 deg..61 4 deg..77 2 deg..03