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 deg., 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 deg. to 16 deg. lower than the temperature of the air; in May, from 4 deg. to 14 deg. lower; in June, from 6 deg. to 20 deg. lower; in July, from 4 deg. to 17 deg.; in August, from 6 deg. to 15 deg. lower; and in September, from 6 deg. to 21 deg. lower. The dew-point is, then, during the important months of vegetation, within about 20 deg. of the temperature of the air. The temperature of the dew-point, as observed by Prof. Bache, was highest in August, 1843, being 66 deg., and lowest in January, 1844, being 18 deg.; in July, 1844, it was 64 deg., and in February, 1845, it was 25 deg.. 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, 50-1/2 deg., air, 57 deg.; five hours after midnight, dew-point, 49 deg., air 54 deg.; three hours after noon, dew-point, 54 deg., air, 63-1/2 deg.. The average temperature for July, August and September, was, at midnight, dew-point, 58-1/2 deg., air, 65 deg.; five hours after midnight, dew-point, 58 deg., air, 62 deg.; three hours after noon, dew-point, 60-1/2 deg., air, 78 deg.. The average temperature for the year was, at midnight, dew-point, 42 deg., air, 48 deg.; five hours after midnight, dew-point, 41 deg., air, 46 deg.; three hours after noon, dew-point, 44-1/2 deg., air, 59 deg.. 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 49 per cent. -- 2 P. M. May 59 " 72 " June 55 " 63 " July 50 " 61 " August 55 " 58 " September 50 " 57 "