1 1 1.-. 



MKTEOROLOGY. 



[HYGROMETERS. 



effect of Tarring amounts of moisture in the atmosphere. 

 Every lady know* that she cannot retain her hair in cur] 

 during w.-t weather, to well as when the weather is dry, 

 because of the moisture present, which caucus the hair 

 pirala to relax and unfold. In point of fact, each hair 

 contract* and elongates alternately, by every mutation of 

 drynea* and moisture ; so that if only the exact ratio of 

 contraction and expansion could bo determined and 

 applied, the meteorologist might construct a hygrometer, 

 having for its basis of actuation, a human hair. Tbe 

 hair-hygrometer of Saussure takes advantage of this 

 principle. 



To construct this hygrometer, a soft human hair is 

 boiled for a short time in a solution of sulphate of soda ; 

 afterwards for a few minutes in pure water ; it is then 

 well washed to free it from all adhering salt, and dried in 

 a shady place. Next, one extremity of the hair is 

 fastened to the extremity of a little tongue, and the other 

 end is wound round a small pulley having two grooves. 

 The second groove is for the purpose of retaining a fila- 

 ment of silk, from which a weight is suspended for the 

 purpose of retaining the hair in a constant state of 

 tension. To the pulley is fastened an index, traversing 

 a graduated arc, whenever the pulley turns in any di- 

 rection by the contraction or elongation of the hair. The 

 graduation of the instrument is thus effected : It is 

 placed in a receiver holding chloride of calcium or con- 

 centrated oil of vitriol ; the air is exhausted from the 

 receiver, and the place where the index then stands is 

 marked. This mark corresponds with the point of 

 greatest dryness, or of the scale. We have next to 

 determine the point of greatest saturation, which is thus 

 effected : The instrument is next placed in a receiver 

 containing a dish of water ; so that as the hair elongates 

 the index turns, and finally coining to rest, the point at 

 which it stands is marked. This mark corresponds to 

 the maximum of moisture, which of course may be indi- 

 cated by any number arbitrarily selected ; this number 

 being usually 100. Finally, it remains to divide the 

 interval between the two extremes into 100 equal parts, 

 and the instrument is complete. Notwithstanding all 

 the care which may be devoted to the construction of 

 this instrument, it is, after all, scarcely deserving the 

 name of a hygrometer it is little better than a hygro- 

 scopo. 



Occasionally, certain helical vegetable fibres have been 

 used as hygrometers, in a way which the accompanying 

 diagram will render manifest. 



Let A (Fig. 62) represent a circular card or other flat 

 disc, and B a vegetable filament heli- 

 cally coiled it is evident that the helix 

 will unravel in a damp, and tighten its 

 coil in a dry, atmosphere. An instru- 

 ment of this kind has been sometimes 

 employed for the purpose of deter- 

 mining whether a bed be moist or dry. 

 The instrument is a very good hygro- 

 Bcope ; but. inasmuch as the coiling and 

 uncoiling of the helix is most compar- 

 able with equal arcs, the instrument cau hardly be 

 termed a hygrometer. 



We are under obligations to the ingenuity of the 

 Dutch for another hygrometer, or rather hygroscope for, 

 like the instrument just described, it is not a true indi- 

 cator of the quantity of moisture present in the atmo- 

 sphere. The instrument is of this kind : A piece of 

 catgut is impended from one extremity, and to the other, 

 or lower extremity, is fixed, transversely, a little hori- 

 zontal bar. On one extremity of the bar, a lady, in gay 

 summer attire, is represented ; on the other a man, 

 drened appropriately for a rainy day : finally, the catgut 

 and it* toy-appendage* are surrounded with a case, hav- 

 ing two openings, and in such a fashion that only one of 

 the toy-images can be visible at a time. Now, it is 

 evident that the fibres of the suspended catgut will 

 partially untwist under the influence of moisture, and 

 re-twit an the atmosphere becomes dry ; whence it fol- 

 low* that the lady will appear under the bitter circum- 

 stance*, and the man under the former. This instru- 



ment, though something more than ingenious for it is 

 a good hygrutcopo doe* not merit the dignified term of 

 hyijrometer. 



The Dew-point Hygrometer of Daniell. If a wine- 

 bottle be taken from its bin, it will frequently be found 

 covered with moisture ; and in proportion as the air is 

 saturated with moisture, so will the depression of tem- 

 perature be at which this moisture begins to be deposited 

 on the bottle. In this manner, if we had the means of 

 regulating the temperature of the bottle at our will, de- 

 pressing it at pleasure, we might ascertain the exact 

 temperature at which moisture would begin to be depo- 

 sited ; and thus, noticing the variations of temperature at 

 different times, we might establish and tabulate a corre- 

 spondence between each particular temperature at which 

 moisture was deposited, and the corresponding amount 

 of moisture contained in the atmosphere. Now, the 

 degree of temperature at which moisture begins to be 

 deposited in this way, is called the dew point ; and hence 

 the propriety of the appellation, dew-point hygrometer, 

 whicli has been given to the instrument presently to bo 

 described. It consists of a doubly-bent, exhausted glass 

 tube, each end terminating in a bulb. One bulb is 

 covered with a coating of thin gold or platinum foil, the 

 other with a fine linen rag. The former bulb is par- 

 tially filled with ether, and holds a small thermometer, 

 the graduated portion of which passes up the tube. If 

 ether be dropped on the second bulb, evaporation rapidly 

 ensues, and the bulb is cooled, thereby condensing the 

 vapour of ether which it contains, and permitting a new 

 evolution from the ether in the bulb. This evolution of 

 ether cools the bulb, and causes dew to be deposited on 

 its surface. The inclosed scale indicates the dew-point. 

 The following reasoning explains how the determination 

 of the dew-point can indicate the amount of aqueous 

 vapour in the atmosphere. In proportion as the tempe- 

 rature of the air is elevated, will it be capable of holding 

 more moisture. Hence, by cooling the air, its power of 

 holding moisture is diminished ; a portion of moisture, 

 therefore, becomes condensed in the form of dew- vesicles. 

 The greater the moisture contained in air, the more 

 readily will condensation ensue for a given reduction of 

 temperature. 



The dew-point hygrometer of Daniell, though a great 

 advance upon the rude instruments just described, is 

 attended with some imperfections. Its construction has 

 been improved upon by Dobereiner and by Regnault ; but 

 the instrument, in its most perfect form, still leaves much 

 to be desired. Not only does its employment necessitate 

 the use of a large amount of ether ; but, what is of more 

 consequence, when the weather is extremely dry, the 

 deposition only takes place with great difficulty. A far 

 more effective instrument, though based on dillerent con- 

 ditions, is that now to be described. 



The Psychrometer. The psychrometer consists of two 

 thermometers mounted on the same frame, the bulb of 

 one thermometer being naked, whilst the bulb of the j 

 other is enveloped in muslin or other similar absorbent 

 texture, from which there extends a wick-like absorbent 

 stem, terminating in a cistern of water. From a con- 

 sideration of the structure of this compound instrument, 

 it will be evident that the mercurial column of the naked 

 or uncoated bulb will stand higher than the mercurial 

 column of the second or wetted bulb. The reason of this 

 is obvious. The process of evaporation lowers the 

 temperature : and it follows that, under one condition, 

 and only one, can the readings of the pair of thermometers 

 which constitute the psychrometer correspond namely, 

 when the atmosphere is saturated with moisture, to such 

 an extent that it is unable to take up more. By an exten- 

 sion of this reasoning it will be now evident that the 

 mercurial readings of the pair of thermometers will 

 continually vary, according to the amount of dryness or 

 moisture of the surrounding atmosphere. The variation, 

 in point of fact, is in an inverse ratio to the amount of 

 moisture ; so that by means of formula; we can easily 

 connect the indications of the psychromoter with the dew- 

 [>oint. 

 Dtwnal Variation of Atmospheric Moisture The 



