225 



Reduced 

 Barometer 

 readings. 



Difference in 

 the height of 

 the mercurial 

 column, at 

 the sea level, 

 and at given 

 altitudes . 



Height, in feet, 



per inch of 



difference in the 



height of 



the mercurial 



column. 



Corrections per 



1° Fah. 



difference in 



temperature. 



Altitudes, in 



feet, above the 



sea level. 



inches. 

 26 



inches. 

 4 



feet. 

 935-9 

 / Diff. 

 | 17-1 



feet. 

 1-9 



feet. 

 3743-5 



27 



3 



918-8 

 / Diff. 

 \ 16-4 



1-9 



2756-2 



28 



2 



902-4 

 / Diff. 

 | 15-5 



1-9 



1804-8 



29 



1 



886-9 

 / Diff. 

 \ 14-9 



1-8 



886-9 



30 







872-0 



1-8 



Sea level. 



"The table in the above form having proved of great service in the author's 

 professional practice, it has been extended for publication, by calculating the 

 altitudes for every hundredth of an inch difference in the height of the 

 mercurial column, from 30 inches to 26 inches ; and a column of temperatures 

 has been added, which will be found of considerable assistance in calculating 

 the difference between the actual and the tabular temperature at any given 

 altitude." 



Mr. Dobson then proceeds to give the principles upon which the tables 

 are framed, at greater length ; with full explanations of the tables themselves, 

 directions for registering the observations, and for using the tables in the 

 calculations of altitudes. 



A chapter is devoted to " General Observations," in which he states that, 

 " in tolerably level country, and in clear, calm weather, the observations may 

 be extended to a distance of from fifteen to twenty miles from a well- 

 ascertained bench-mark without risk of serious error. If, however, there is 

 much wind, not only must these limits be greatly reduced, but it will be 

 advisable that the observations at each of the upper stations should be twice 

 repeated at ten minutes intervals, in order that it may be ascertained whether 

 the barometer is rising or falling, and that the index error may be adjusted 

 according to the directions whence the changes come. 



" It must, however, be remembered that the fluctuations of the barometer 

 due to variations in the quantity of aqueous vapour in the atmosphere, as 

 well as to other causes, are so great as to render all barometric observations 

 valueless, as engineering data, which cannot be corrected for the deviations 

 from mean atmospheric pressure, by comparison with a register kept at some 

 neighbouring station, of which the altitude has been ascertained." 



The author suggests that " although the mercurial barometer should always 

 be used, when practicable for the observations at permanent meteorological 

 stations, it is at once too cumbrous and too fragile for the rough work of a 

 reconnaissance survey. For this purpose a properly compensated aneroid 

 barometer may be substitvited, with advantage, for the more perfect instrument. 

 Up to the present time, the use of the aneroid barometer has, with trifling 



