urn 



UOLOGV. 



[OBSEUVATIO.NS. 



nil, direction and force of winds, and state of tho 



wea'h.r. Let UH further supjMWo tli.it two ol.-ei v.itiuns 

 aru tftkou daily, tin- liuiirs of observation being A.M. 

 and In i M 



I'tvmire ofOu Air. Tho baroiu instru 



nit-Hi by tthicli tin- alteration* in the weight of tlio air 

 ire MourUiufd. Tim gravitation ..f tin- earth exerts a 

 :.ich would always bo alike, wen- it 

 nut Unit lateral disturbances lial tin- j ...\- i- >;' removing 

 a }Hrtiun of that pressure, and adding it 

 Tims, suppose the average pressure to be 2ii inches ; if 

 ii.-- la!oii,i:ter be seen to rise to 30 inches, it is certain 

 ih.it, in another portion uf tin- earth, a corresponding fall 

 has taken place to a i ..nut I'm- this change : Luwerer, as 

 it is not our object to enter into the physical and local 

 changes of the. weather here, we shall at once proceed to 

 describe the reductions that are requisite. 



It is essential to correct observation, that the baro- 

 d bo a standard instrument. The ordinary 

 instruments are useless, owing to the friction of the 

 ineivury ,-i-.-.iint the sides of the glass in small tubes ; the 

 impossibility uf applying the reduction necessary to 

 Cot the alteration of the height of mercury in the 

 cistoru, with the common wheel barometer, owing to a 

 rise and fall in the tube ; and, further, bocau.su if errors 

 occur in this barometer, they are increased by the cir- 

 cumstance of the index of the wheel barometer being ;i 

 lung arm worked by a small wheel, thus multiplying all 

 the errors. 



The Standard Barometer should have a tube of not less 

 thau i,,ths of an inch in diameter ; and the nearer it 

 approaches to i*,tli.s the better, as the correction requisite 

 for the friction against the sides of the glass, rapidly 

 diminishes with an increase in the size of the tube. This 

 is the correction for capillarity, which is additive. The 

 base of the tube should be plunged into a cistern of pure 

 mercury, aud the scale of inches should hare a rod at- 

 tached, terminating in an ivory point, so contrived as 

 to be moved by rack-work until the point jiut touches 

 the surface of the mercury. This is requisite, in order 

 that the measurement may be made from the surface of 

 the quicksilver in the cistern. Suppose we either neglect 

 this operation, or that the barometer is one not capable 

 of having it applied ; and let us further take a reading 

 without reference to this correction, after a sudden change 

 in the barometer ; this reading may be represented as 

 30"036 inches. On bringing the ivory point down to the 

 mercury, this second reading may be 30 '074 inches : 

 thus exhibiting an error, from the want of this pre- 

 caution, of "038, or nearly four hundredths of an inch. 

 The thermometer fitted for use should have its bulb 

 plunged into the cistern of mercury, otherwise it will 

 not give the temperature of the mercury itself, but 

 merely show the heat of the apartment in which it is 

 pbewL Such an instrument is produced by many of 

 the loading London makers : of similar kind are those 

 for the Royal Society, Greenwich Observatory, 

 Admiralty, and the liritish Colonial Observatories. 



Detcription.a (Fig. 69) is the mahogany board to 

 affix against the wall ; 6, brackets which support the 

 barometer, between which it is capable of being revolved 

 so as to observe the light on the surface of the mercury ; 

 c, a vase, which unscrews, to allow of the socket d being 

 removed, to receive the upper end of the barometer ; e, 

 adjusting screws for shifting the lower centre, by 

 wliich the barometer is to be made exactly perpendicular: 

 to accomplish this, the ivory point is to be adjusted to 

 tho surface of the mercury ; the barometer gently turned 

 between the two brackets ; and if, in any position, the 

 jxiiiit should bo elevated from tho surface, or depressed 

 nit i the mercury, the screws must be altered accordingly, 

 until the point coincides in every position ; /, the key by 

 which the ivory point is adjusted the ivory point being 

 a termination of the brass scale marked off at tho ti-i 



- , and which is adjusted by means of a tan- 



. the glass part of the cistern, tin 

 which tho surface of the mercury and ivory point 

 seen ; A, th<- .-i-t. in . i, tlio screw, which is to be |uo.,ened 

 when the barometer is lixed, to admit the atmospheric 



pi-ensure ; k, the movable part of the cistern, on which 

 tho mdux^f is oujj'ravud ; I, the key by which the Vernier 



fig. Mk 





is adjusted ; m, the thermometer dipping into, and show- 

 ing the temperature of tho mercury. 



This barometer is necessarily expensive. Modifications 

 of this standard are made by many London iustrument- 

 makers ; aud these are more reasonable in price, answer- 

 ing remarkably well, though possibly not equal to Mr. 

 Newman's standard. Having procured a barometer, it 

 should be compared with the Standard, in order to ascer- 

 tain its index error. The apartment in which it is kept 

 must not be subject to great changes of temperature ; 

 one with a window facing the north is to be prefei 

 aud the instrument should hang where there is a strong 

 light, but an outer wall should be avoided. 



TIIK.I;MO\II.TKRS. There are various constructions of 

 instruments for ascertaining the temperature of the air ; 

 of these tho mercurial ones are the best. Thermometers 

 For comparison require to be placed upon a proper stand, 

 as the errors arising from peculiarity of situation, from 

 radiation, absorption of heat, \'c., will alter the reading 

 very materially. There are two forms of stand tho one 

 constructed by the late Henry Lawson, Esq., F.R.S. ; tho 

 other by James GUwhcr, Esq., F.R.S. 



