MISCELLANEOUS PHENOMENA.] 



METEOROLOGY. 



1165 



HygionuiUr. Another instrument for as- 

 certaining the dew-point from direct observation. It 

 differs considerably in its construction. Some meteoro- 

 logists prefer it to Daniell's hygrometer. Messrs. Negretti 

 and Zambra have constructed a modification of Reg- 

 iiault's hygrometer, and from them it can be procured 

 at the ame price as that usually charged for Daniell's 

 hygrometer. 



Council's Hygrometer is a third instrument for ascer- 

 taining the dew-point from observation. In this, the 

 temperature is lowered to the dew-poiiit by means of an 

 ex liausting syringe. The wet and dry bulb thermometer, 

 however, answers every purpose, and, to the ordinary 

 observer, is not so liable to be incorrectly read off. 



The Cyanometer consists of a flat ring, divided into 53 

 equal parts, and numbered from to 52, the being 

 white, and 52 very dark blue ; the other numbers are 

 painted every intermediate tinge from nearly white to 

 deep blue. By its use, the colour of the sky can be com- 

 paratively recorded. 



GENERAL DiRtcrioNS. The following occasional phe- 

 nomena require a few words. The most uniiistructed 

 observer may give useful information on these subjects 

 wit.i conipaiatively little trouble to himself, by making 

 known the particular features to be observed. It is 

 desirable to note, in 



Thunder-storms, the direction in which they move, the 

 point of the horizon in which they were first noticed, 

 and that in which they disappeared ; the time when 

 thunder was first and last heard ; the colour of the 

 lightning; the number of seconds elapsing after a flash 

 l^f ore thunder became audible (noted at different periods 

 during the storm's continuance) ; the commencement 

 and termination of rain ; the direction of the wind be- 

 fore, after, and during its continuance ; the time when 

 the electrical breeze springs up (this is a peculiarly violent 

 breeze noticed in nin.it thuuder-storms) ; whether hail 

 falls ; and any other feature that may appear remarkable 

 or be deemed desirable to be recorded. 



Aurora Borealii. Its position amongst the stars, 

 whether merely a low auroral arch, or accompanied by 

 coruscations. If a brilliant display, whether a cupola 

 or dome is formed, a little south of the zenith ; and if 

 formed, whether oscillatory among the stars. The hour 

 of its occurrence when seen as a diffused light ; if there 

 be floating patches of luminous haze or cloud, &o. 



,Si,/n nii'l. l.iti.nr Unli*. When visible, the quarter pf 

 the heavens where they appeared, and how long they 

 remained visible. 



Mock Sum and Complicated Circlet of Light. Their 

 form and position with respect to the sun or moon; 

 whether prismatic. 



Meteon or falling Stars. Their apparent size, shape, 

 colour, path amongst the stars, velocity and duration ; 

 whether accompanied by a streak of light, or separate 

 fragments ; if large, whether a streak pf light remained 

 after the meteor itself had disappeared ; and after burst- 

 ing, whether any noise of explosion be heard if so, how 

 many seconds after the meteor itself had burst ; the time 

 of appearance, etc. These observations should mpre 

 especially be attended to from the. (itli to the ICth of 

 August, and from the 9th to the 14th of November.* 



Gales of Wiivl. Their direction, and estimated force ; 

 when they commenced and terminated ; the height of 

 the barometer during their continuance. 



Snow. When it fell, how deep in inches on the 

 ground, and the form of the snow-crystals. To sketch 

 the crystals, a magnifying glass is requisite. 

 Hail. The shape of the stones, &c. 

 The times of breaking up of long dry periods, and 

 frosts ; the termination of rainy periods ; the commence- 

 ment and duration of fogs, wind-changes, <tc. 



fur Eclipses. During their continuance, and before 

 and after, record the temperature in sun and shade re- 

 peatedly. Expose for ten seconds, every five minutes, 

 ulipA of sensitive paper, to ascertain the effect of the 

 diminution of sunlight. 



During the period*, the appearance of "falling tar" in nlwaya 

 found to be mure frequent than during other tlmet of the year. Ku. 



REQUISITE TABLES os REDUCTION. 1st. Glaisher's 

 Ilij'irometrical Tables (2nd edition). These splendid 

 tables enable the observer, with comparatively little 

 labour, to calculate the temperature of the dew-point ; 

 the elastic force of vapour ; the weight of vapour in a 

 cubic foot of air ; the additional weight of vapour required 

 to saturate a cubic foot of air ; the degree of humidity ; 

 the whole amount of water in a vertical column of the 

 atmosphere ; the weight of a cubic foot of air ; and also 

 to separate the pressure due to vapour from that due to 

 the gases, for any temperature from 10 to 100 F. There 

 is also a table for reducing the readings of the barometer 

 to the level of the sea. Mr. Glaisher has calculated this 

 table from the fact determined by M. Regnault, that air 

 expands njhnth part for every increase of 1 of heat. 



2nd. Tables of the Corrections for Temperature to Re- 

 duce Observations to 32 F. for Barometers with Brast 

 Scales, by 3. Glaisher, Esq. It is absolutely requisite to 

 reduce the readings of the barometer to a certain acknow- 

 ledged temperature, otherwise the true pressure of the 

 air could not be ascertained ; for it must be remembered 

 that, besides the pressure of the air on the mercury, the 

 mercury itself obeys the same law which is pointed out 

 to us by the thermometer i.e., it expands by heat, and 

 contracts by cold. Thus, suppose the actual pressure of 

 the air to be stationary, the barometer will be seen to 

 rise or fall, if there be an increase or decrease in the 

 temperature of the air. In like manner, the metal scale 

 of the barometer is subject to expansion and contraction 

 I by an increase of heat or cold ; and this, as Mr. Glaisher 

 I says, explains the apparent anomaly that, although the 

 readings are said to be reduced to 32 (or the freezing- 

 point), the point of no correction is 28^. As metal 

 bars will vary in length with every degree of tempera- 

 ture, it is apparent that a certain temperature should be 

 determined upon at which the standard unit of measure 

 must be referred to. 



Now this temperature has been fixed at 62" F. ; 

 therefore above 02, as the metal will expand, this will 

 make the divisions on the scale of the barometer too 

 large, and consequently the barometer will read lower 

 than it should do ; on the contrary, below 62, the metal 

 contracting, will bring the index divisions closer to- 

 gether, and the barometer will read too high, unless 

 corrected. Thus, owing to this cause alone, at the tem- 

 perature of 32, the barometer is made to reach '009 of 

 an inch too high. 



The great use of the reduction for temperature will 

 be at once apparent, when an example is given : thus, 

 suppose a person has two barometers, one in a room 

 heated artificially, and the other as cold as possible : 



Reading of barometer at temperature of 90 = 30-200 

 Correction for temperature . . . O'lt>5 



Reading corrected for temperature . 



30-035 in. 



Reading of barometer at temperature of 40 = 30.066 

 Correction for temperature . "031 



Reading corrected for temperature 



30-035 in. 



The difference -134 inch between the two readings, being 

 due to the expansive action of heat. 



These tables have been calculated trom Schumacher's 

 formula, which is here copied : 



m(t 32) s(t 62) 

 r+ro(t 32) 



2 = reading of barometer. 



m = the expansion in volume of mercury for 1 F. = 

 0-0001001. 



t = the temperature of the mercury and the scale. 



,1 = the expansion of the brass scale in length for 

 1 F. = 0'000010434 (the normal temperature being 62^). 



