Page:The American Cyclopædia (1879) Volume VI.djvu/70

 DEW DEWEES tin- other hand, a strong wind conveys too inm-h ln-at to all>v of the moling of the be-
 * !..idy. 'I lu- hy^rometric tables of Glai-

iilt, and others, alh>v of an accu- vdiction as t< the quantity of dew that i. and even enable one to de- termine uhcther frost or dew will prevail. It will thus be seen that the true origin and ..f d.-w was quite misunderstood until .1 the I'.ith century. The DEW POINT is the tempi- nit 11 re at which the vapor contained in the atmosphere condenses into water. It depends upon the amount contained within a -pa<-c ; the greater this is, the higher will he the degree of heat necessary to retain it in a vaporous condition. When the air is saturated with vapor it is of course at its dew point, and any reduction of its temperature will cause the separation of a portion of mois- ture. As this process continues the dew point becomes gradually lowered, and although the proportion of moisture held in solution by the uir diminishes, the degree of humidity remains the same ; for this term is used not to denote the actual proportion of vapor, but the power of sensibly manifesting itself upon any given reduction of temperature. It is often called relative humidity, and is expressed in parts of 100, or the amount which would saturate the air at the particular temperature when the observation is taken. Thus, at Philadelphia the average humidity of the air is 73, meaning 73 per cent, of the quantity necessary for sat- uration ; at St. Helena it is 88 ; at Madrid, 62 ; in parts of India it is sometimes as low as 10; and on the Andes it is often still lower. Air which is heated much above its dew point is regarded as dry, although it may contain a greater proportion of vapor than that which is called humid. In California the dew point is .sometimes 78 below the temperature of the air. and among the Andes the difference is often great. T. It has been found as much as 46 at Philadelphia, but the usual range is from 10 to 25. The following table shows .n ivlative humidity of the atmosphere in New York from several observations made e.ich day at the Cooper institute, for the Smith- sonian institution, by Prof. Morris: Jan. INI 8? OM BT-fl IfrM U i7.: H IT-M 41-88 Ml M M '
 * ..],-.

M M Data, Relative Temperature, Humidity. Fahrenheit. June 4840 B0-M 54 .M"_>i> 1848 61-48 59-1 fi 46-86 49-1 76 72-66 61-88 66-9 88-16 7OC

i<;-<;<; The litr.-r.-nn. 1,,-tween the dew point and the temiH-ratiire of the air is called thecomplement lew point. From numerous ohserva- tions which have been made with Daniell's and Bache's hygrometers, and with the wet- hulb thermometer, a method has been deduced for determining the dew point with sufficient accuracy, by observations made with the lat- ter instrument. The ratio of the complement varies with the temperature of the air : thus, when it is 53 the difference between the dry and wet bulb is one half the complement ; at 32 it is one third, and at 26 only one sixth. Tables have been constructed for readily de- termining the dew point, which may be found in the various works on meteorology. Thus, in the above table it is stated that the relative humidity of the air at New York, June 1, 1873, was 53'13, while the temperature was 76 F. By referring to the table of relative humidity in the article HYGEOMETET, it will be seen that the complement of the dew point was about 18 ; that is, the dew point was about 18 lower than the temperature of the air, or about 58. (See EVAPORATION, HEAT, and HYGEOMETEY.) DEW, Thomas Roderick, an American publicist, born in Virginia, Dec. 5, 1802, died in Paris, Aug. 6, 1846. He graduated at William and Mary college, and afterward travelled for two years in Europe. In 1827 he was chosen pro- fessor of moral science in William and Mary college, of which he became president in 1836. In 1829 he published " Lectures on the Re- strictive System," and in 1833 an elaborate essay on " Slavery," which is said to have prevented emancipation in Virginia at that time. He died suddenly while on a visit to Europe with his bride. His most elaborate work is U A Digest of the Laws, Customs, Manners, and Institutions of Modern Nations," which appeared in 1853. DEWBERRY. See BKAMBLE. DEWEES, William Potts, an American physi- cian, born at Pottsgrove, Pa., May 5, 1768, died in Philadelphia, May 18, 1841. He at- tended several courses of lectures at the .uni- versity of Pennsylvania, and, without having taken any degree, in 1789 commenced the practice of medicine at Abington, Pa. The de- gree of M. D. was subsequently conferred on him by the university. The yellow fever hav- ing in the summer and autumn of 1793 thinned the ranks of the physicians in Philadelphia, in December of that year Dr. Dewees removed thither, selecting obstetrics for his specialty, and achieved a high reputation in that depart' ment. In 1812, being threatened with 'a pul- monary affection, he relinquished practice, and for five years devoted himself to agriculture. In 1817 he returned to Philadelphia. In 1826 he was elected adjunct professor, and in 1834 professor of obstetrics and diseases of women and children in the university of Pennsylvania. In the latter year he was attacked by'paraly- sis, and at the commencement of the follow- ing year he was obliged to resign his profes- sorship. He settled in Mobile, but resumed his residence in Philadelphia about a year