HYGROMETRY HYL^EOSAURUS 133 top of each, both opening into the hollow stand that supports the tubes. A pipe for exhausting the air by means of a sort of bellows or the flow of water connects with the hollow in the stand by an opening near its base. The two tubes are closed, each with a cork through which a thermometer tube is fitted, the bulb in one reaching nearly to the bottom. Over the lower end of this one a very thin and highly polished thimble of silver nearly two inches long is fitted, and a fine tube open at each end is passed through the cork, reaching from the external air nearly to the bottom of the tube. Ether is poured into this bulb, covering the lower end of the thermometer, and rising an inch or two higher than the upper edge of the silver thimble. To determine the dew point, the apparatus for exhausting the air from the hollow stand is set in action. This causes the air to pass through the fine tube, and bubble through the ether, keeping it in motion and taking up its vapor. The liquid, the thermom- eter bulb, and the silver coating of the tube equally feel the reduced temperature, and the instant this readies the dew point, the whole surface of the silver is covered with moisture. The temperature of the thermometer placed in the ether is then observed, while the other marks the temperature of the air. By stopping the current of air the temperature rises, and the moisture disappears from the silver. The ther- mometer is to be noted again, and the mean of the two observations taken for the dew point ; or several trials may he made in rapid succes- sion. To avoid affecting the result by the warmth radiated from the body, a small tele- scope may be used in reading the thermometer. The instrument has been modified by Prof. Connell in substituting for the tube a small flask of highly polished brass or silver, into the neck of which is secured an exhausting syringe. The second of the two methods above referred to, by which the humidity of the air is ascertained, involves the determination of the temperature of evaporation ; and the instrument used is the wet-bulb thermometer or psychrometer invent- ed by Prof. August of Berlin, and described in his work Ueber die Fortscliritte der Ilygro- metrie (Berlin, 1830). It consists of two deli- cate thermometers placed near together. The bulb of one is covered with muslin, which is kept wet by water supplied from a vessel close by through capillary conduction. The instru- ment is placed in a light draught of air, and as evaporation goes on the mercury in the wet- bulb thermometer sinks to a certain point ; the temperature of both is then noticed. If the air was nearly saturated with moisture, the differ- ence will be found to bo very slight. The baro- metric pressure is observed at the same time, and data are thus afforded for calculating the elastic force of aqueous vapor in the air. The formula for this calculation, modified by Re- gnault, and the psychrometrical tables deduced from it, are given in the volume of tables re- ferred to above, and are equally applicable to the estimation whether the dew-point instru- ment or wet-bulb thermometer is used. To render them more convenient, they have been converted by Prof. Guyot into English measures. Tiie series also contains tables of the weight of vapors in a given space at different tempera- tures. The method by the wet bulb, though regarded as decidedly the most convenient means of determining the elastic forces of the vapor, and thence the humidity of the air, is still rendered somewhat uncertain in its results from the impossibility of keeping the wet bulb uniformly moist, and from other causes also. The uncertainty of its results is indeed in some cases so great that Regnault in 1872 recom- mended that, for accurate meteorological pur- poses, resort should be uniformly had to the chemical methods of extracting and weighing the aqueous vapor in a given volume of air. To this end he has devised a simple arrange- ment by which concentrated sulphuric acid may be exposed to the atmosphere and absorb its aqueous vapor; a method that is specially ap- plicable at very low temperatures. The ulti- mate object of these hygronietrical investiga- tions is, by enabling the meteorologist to ascer- tain at all times, in all localities, and at all ac- cessible elevations, the true condition of the atmosphere as to moisture, to furnish him with accurate data for studying the laws which con- trol its variations. The following table of rela- tive humidity is prepared for every 5 F. from 5 to 95 above zero, and for a difference of temperature between the air and the dew point, technically called the complement of the dew point, ranging from to 18. (See DEW POINT, in article DEW.) TABLE OF EELATIVE HUMIDITY OF THE AIE. TEMP. OF AIR. DIFFERENCE OF TEMPEKATURE OF THE AIR AND OF TIIE DEW POINT. 2- 3 4 5;6789 10 12 6 1009691 878880,76726966 10 10096918768807678,7066' 15 100'96;91 878880,7678 70 66' 20 100l96!91 87 88 80;76 78 69 66 25 1009619187848076787067 80 IlOO 96J92 88 84 8l!7T 74 70 67 85 100 96i92 88 84 82 77 74 71 68 40 IlOO 96,92 89 86 82 78 75 72 69 45 IlOO 96'98 89 85 83'79 75 73 70 50 IlOO 96 98 89 S6 68 'SO 76 74 71 65 10096:9890868880777472 60 100 96:98 90 86 84 81 77 75 72 65 100 97 98 90 87 84 I 81 78 75 72 70 Il0097l93 90878481 787673 75 1009719491 878482797673 60 100 97|94 91 886682 797774 85 IlOO 97!94 91, 88 85 88 8n 77 74 90 10097194918886888077,75 9.") 10097,949188858880:78,75: 52 47 48 63 47 48 ! 4S 44 68 , 48 44 61
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59 go 68 I 57 I 63 53 48 64 '49 52 64 59 , 55 1 51 64 ' 60 56 , 70 ' 65 ' 61 i 66 71 65 j 61 ] 57 71166 62 5S 72 ; 67 I 62 68 72 ; 67 ! 63 69 78 j 68 , 63 59 78 68 ' 64 60 IIYRSOS, or Shepherd Kings. See EGYPT, vol. vi., p. 460. Hl'LJCOSAURCS (Gr. i^aZof, belonging to wood, and aaipof, lizard), the name given by Dr. Man- tell to an extinct dinosaurian reptile, from the Jurassic strata of Tilgate forest, having the usual mammalian characters of its tribe, viz. : long bones with a medullary cavity, pachy 7
