Mr. Meikle on an Air Barometer. 215 



minutely described, I have somehow either undervalued them 

 or failed in discovering what they are ; for the common 

 portable barometer seems still to be the more convenient of 

 the two, especially considering the very troublesome correc- 

 tions his instrument requires for change of temperature, &c. 



In Mr. Murray's barometer, the rise of the mercury, on 

 being carried to a higher station, will, in general, be less than 

 the corresponding depression in the common barometer. Its 

 sensibility will therefore be less ; and unfortunately there does 

 not appear to be any means of remedying this defect, which 

 is occasioned by the following causes : The included air, hav- 

 ing most likely become colder on reaching the upper station, 

 will be less elastic, and the suiking of the mercury in the 

 cistern, occasioned by its rise in the tube, and by its contrac- 

 tion from cold, must enlarge the space occupied by the air, 

 and still further diminish its elasticity, which will not, there- 

 fore, be able to raise the mercury to the proper height. Some 

 inaccuracies, it is true, may be lessened by sliding the tube ; 

 but the whole of the requisite corrections could be more easily 

 estimated were the tube fixed; for if the bulk of the glass tube 

 within the cistern be varied, a new source of error will be 

 inti'oduced. 



In order to correct this instrument for a change of tem- 

 perature, we must find — the change in the length of the 

 mercurial column — the change in bulk of mercury in the 

 cistern — and the change in the elasticity of the included 

 air. Indeed, the elasticity of the air would require cor- 

 rection, although the temperature were constant, on ac- 

 count of the variable bulk of mercury in the cistern. It also 

 seems highly probable that the included air, having so large 

 a proportion of its surface in contact with mercury, will be of 

 a temperature intermediate between that of the mercury and 

 the external air, when these disagree. 



In its present form, I do not see how a vernier scale can 

 be applied ; and without this, the instrument can be of little 

 use in measuring altitudes. Perhaps Mr. M. may hereafter 

 be able to alter it so as to admit of this indispensable appen- 

 dage. On the whole, I should think his instrument would 

 liave been more convenient, though at the same time more liable 

 to fracture, had the tube first branched out from the bottom 

 of the cistern, and then turned round till it stood upright. 

 In this form, a vernier could be applied, and the corrections 

 for temj)erature, &c. could be as easily ccmputed. 



The construction of a more convenient barometer is a sub- 

 ject to whicii I have often turned my attention ; but have not 

 yet succeeded altogether to my wish. The following, how- 

 ever, 



