797 



HYGROMETKY. 



HYMEN. 



798 



place of F, and 30 inches for B, we get '00205 for the number of grains 

 of moisture in a cubic inch of air corresponding to the elasticity F'. 

 To this number corresponds the temperature 44 16' ; which may be 

 found by inspection in a table formed to contain the Tames of o for 

 different degrees of temperature. 



The subject of hygrometry, including the principle and construction 

 of the numerous instruments which have been employed for the pur- 

 pose of determining the amount and tension of aqueous vapour in the 

 air under various circumstances, has formed, during a century past, 

 so considerable a part of meteorology and experimental physics espe- 

 cially from the time of the publication of a celebrated paper by De Luc, 

 in the 'Philosophical Transactions' for 1792 and is so intimately 

 connected also with the history of the progress of science in that period, 

 that we have thought fit to retain, with a few slight insertions and 

 alterations, the preceding portion of this article as it originally appeared 

 in the first supplement to the ' Penny Cyclopaedia,' in the year 1846. But 

 it has already been remarked, in the article DEW-POIST, that for the 

 determination of that critical temperature and its depression, the 

 of all hygrometry, the simultaneous observation of the dry-bulb and the 

 wet-bulb thermometer is now almost universally adopted, according to 

 a method we shall proceed to describe. 



In this method the bulb of the wet thermometer, as described by 

 Mr. Glaisher in the Introduction to the 'Greenwich Meteorological 

 Observations' for 1847, is covered with a piece of fine muslin; 

 immediately under it is placed a small cistern of rain-water. A 

 piece of cotton lamp-wick is connected with the muslin, and its end 

 dips into the cistern of watery the water ascends the wick by capillary 

 action, and keeps the muslin on the thermometer constantly wet. In 

 frosty weather the muslin is moistened for a sufficient length of time 

 before each observation, in order to allow the water to have become 

 frozen, and the evaporation from the surface of the ice to have 

 commenced, at the time of making the observation ; the evaporation 

 of the ice will depress the temperature, as if it had remained liquid. 

 These thermometers should be placed and observed in such a locality 

 as shall afford the best chance for procuring a fair indication of the 

 moisture of the general atmosphere, and by no means in any confined 

 or ill- ventilated situation, or where many persons habitually congregate, 

 or which, from any other cause, is usually or periodically damp. The 

 whole apparatus should be protected by a cap of wire-work to defend 

 it from injury, which, however, if it interfere with the readings of the 

 thermometer, should be removed a quarter of an hour before the 

 observation. At night the thermometers should be completely screened 

 from the sky, so as to annihilate all loss of heat by upward radiation. 



The dry thermometer is to be fiftt read, and in reading it, as in all 

 readings of such instrument*, the observer should avoid touching, 

 breathing on, or in any way warming it by the near approach of his 

 person ; and in night observations particular care should be taken not 

 to heat it by approach of the light. The quicker the reading is done 

 the better. The observer will then enter the simple readings, but at 

 the head of each column place the zero correction (with its proper 

 sign) required for it* thermometer, and leave a blank column for the 

 hygrometric depression, or depression of the dew-point, in calculating 

 which subsequently the zero must be applied. 



The reduction of the observations to derive the elastic force <>f 

 at tlte deic-point is effected by the formulae of Dr. Apjohn : 



d h d h 

 F = /-88'30 (a); F= /-98-30 < 6) ' 



(a) To be used when the reading of the wet thermometer is above 

 32, and (h) when below. In these formula; d is the hygrometric 

 depression, It the height of the barometer, / the elastic force of vapour 

 for the temperature shown by the wet thermometer, to be taken from 

 an appropriate table of the elastic force of aqueous vapour for every 

 degree of temperature through a sufficient range, and r the elastic force 

 of vapour at the dew-point, which (all other quantities being known) 

 these formulae enable us to calculate. If, with F so calculated the 

 same table be entered under the column of force of vapour, the corre- 

 sponding temperature is the dew-point, which, however, is not wanted 

 to be known except for objects distinct from that of this process of 

 hygrometry. In all these calculations Mr. Glauber's ' Hygrometrical 

 Tables,' which form a separate publication (second edition), will be 

 found of great value, including as they do all the requisite data and 

 corrections. 



Among these tables, which were originally published in the ' Intro- 

 ductions to the Greenwich Meteorological Observations,' is one which 

 shows the elastic force of vapour, in inches of mercury, for every tenth 

 of a degree, from to 90, calculated from the experiments of Dalton, 

 and those of the late Dr. Ure, noticed in the preceding part of this 

 article. The history of this table we shall briefly state, as it involves 

 the verification of the quantities now generally employed in the re- 

 'liioMiin of hygrometrical observations, as well as that by implication 

 of Dr. Apjohn's formulae already given. Before deciding upon the use 

 of the table, many comparisons were made by Mr. Glaisher between the 

 observed dew-point, and that deduced from the observed temperature 

 of evaporation by means of the formulae of Dr. Apjohn (originally 

 published in the ' Proceedings of the Koyal Irish Academy,' for 1840), 

 lining the values of the elastic force of vapour as given in the ' 1!< -pm t. 

 "f tli> 1'liy-qrn including Meteorology' of thf l!oy:il 



Society of London, 1840 ; and also between it and that deduced from 

 the values of that force and the formulae given by Professor Ksemtz, in 

 his work on Meteorology. It was found in this comparison that the 

 errors of the inferred dew-points were considerable with both sets of 

 tables. Similar comparisons were made, using the table in question, 

 and the errors were found to be always small. Dr. Apjohn's formulas, 

 combined with it, gave results in close accordance with direct observa- 

 tions of the dew-point. It follows therefore that it represents with 

 considerable accuracy, the relation between the tension and the tem- 

 perature of steam. 



We must not conclude, however, while on this branch of the subject, 

 without directing the attention of the reader and hygrometrical 

 observer, to the following remarks of Sir John F. W. Herschel : " The 

 discussion respecting the formulae and coefficients of reduction of 

 observations of this nature can hardly be regarded as satisfactorily 

 terminated ; and it cannot be denied that great difficulty still subsists 

 in determining, by any mere reading of instruments, the exact hygro- 

 metric state of the air. In the absence of direct observation of the 

 Dew Point, the actual absorption and weighing of the water contained 

 in a given volume of air seems to be the only method free from 

 theoretical objection, and it might not be very difficult to contrive a 

 portable apparatus for this purpose." ' Meteorology," in ' Admiralty 

 Manual of Scientific Enquiry,' 3rd edit., 1859, p. 134, note. 



The ' Greenwich Magnetical and Meteorological Observations,' for the 

 years subsequent to 1847, have appeared in an Appendix (also issued 

 separately), to the annual volume of ' Astronomical Observations 

 made at the Royal Observatory.' The hygrometrical deductions are 

 stated for each month among the results of the ordinary meteorological 

 observations, under the head of ' Humidity of the Air,' and consist of 

 the temperature of the dew point, the elastic force of vapour, the 

 weight of vapour in a cubic foot of air, the degree of humidity, and the 

 weight of a cubic foot of air. From August 24, 1856, hourly observa- 

 tions have been made once in every week during tweuty-four hours, 

 the hygrometrical deductions from which, stated for each hour, include 

 also that of the vapour required to saturate a cubic foot of air. Those 

 deductions, together with the other meteorological elements, are 

 further given in the form of monthly means. It is explained in the 

 ' Introduction to the Observations for 1857 ' (the latest which have 

 been published) that, the dew-point having been inferred exclusively 

 (as noted in the preceding article) from simultaneous observations of 

 the dry -bulb and the wet-bulb thermometers ; in order to find the 

 difference between the reading of the former and the dew-point, the 

 difference between the dry-bulb and the wet-bulb readings has been 

 multiplied by a factor taken from the following table, which has been 

 deduced by Mr. Glaisher from the comparison of all the simultaneous 

 readings of the dry-bulb, wet-bulb, and dew-point thermometers from 

 the yew 1840 to the end of the year 1854. The dew-point having 

 been thus found for each individual observation, the mean has been 

 taken for each day (as defined from midnight to midnight), and this 

 mean is corrected by application of the elements in Mr. Glaisher's 

 paper ' On the corrections to be applied to the Monthly Means of 

 Meteorological Observations taken at any hour, to convert them into 

 Mean Monthly Values ; ' ' Phil. Trans.' 1848, part i. 



TABLE OP FACTORS BY WHICH THE DIFFERENCE OF READINGS or THE DRY-BULB 

 AXD WET-BULB THERMOMETERS is TO BE MULTIPLIED, IN ODDER TO PRODUCE 



THE DIFFERENCE BETWEEN THE HEADINGS OF THE DRY-BULB AND DEW- 

 POINT THERMOMETERS. 



(CLOUD ; DEW-POINT ; DEW ; EVAPORATION ; HAIL ; HOAR-FROST 

 METEOROLOOT ; RAIN; SNOW; VAPOUR.) 



HYMEN, or HYMEN^EUS, the god of marriage, was the son of 

 Apollo and a muse. In very early legends he was described as a mortal 

 of extreme beauty, who delivered from captivity Rome Attic virgins 



