METEOROLOGICAL COMMITTEE. 141 



over; which gave a specific gravity 13.628. It was then 

 carefully dried and well filtered. 



The necessity of recording the specific gravity of the mer- 

 cury I think has been somewhat overlooked: but when it is 

 recollected that the difference between 13.625 and 13.626 in 

 specific gravity would be attended with a variation of 0.0022 

 inch in the altitude of a column of thirty inches, its impor- 

 tance is manifest. 



The specific gravity of the mercury in the standard barome- 

 ter of the Royal Society, as ascertained by Mr. Faraday, is 

 13.624, which would create a discrepancy of near 0.009 inch 

 in absolute altitude between that instrument and mine, the 

 barometer standing at thirty inches. 



Faraday, Turner and others, give the specific gravity of 

 mercury at 13.568, and if mercury of this weight should be 

 used in the construction of a barometer it would give rise to a 

 difference of over one-tenth of an inch between such an 

 instrument, and that of the Royal Society and the one now 

 described. The specific gravity of the mercury if given, 

 however, at the time of construction, admits of an easy cor- 

 rection for the purposes of comparison. 



After introducing the mercury into the tube, it was well 

 boiled thioughout its whole length. 



The measurement for the scale was laid off from a standard 

 mean yard, traced by Mr. Hassler. 



For the utility of the application of the platinum guard, I 

 must refer to the interesting Essay of Professor Daniell on 

 the deterioration of barometers, as simply to state the facts 

 coimected with this subject, would occupy too much space in 

 this description. 



The adoption of the micrometer reading of the scale, I 

 think will be found convenient, particularly for observers 

 unpractised in the use of the vernier in finely graduated 

 instruments. 



On proposing the application of the glass plate as a hori- 

 zontal boundary for the surface of the mercury of the cistern, 

 although satisfied myself that no appreciable interference with 

 the free flowing of the mercury would arise from it, in the 

 measurements I had selected, it was thought by some of the 

 Committee that such might be the case. After completing 



