144 



under 200 feet, the process is simply tliis — Place the aneroid or mcrcarial baro- 

 meter (if the latter its readings must be corrected for temperature) level with 

 the rain gauge. Kcad it, say 30 '17, then place it level with the bench mark and 

 read it, say 30 '01, then take it back to the gauge, and read it again, say 30"15. 

 All that is necessary is to take the mean of readings at the gauge. 



135 feet. 

 and multiply the difference in hundredths between the mean at the gauge and 

 that at the bench-mark by 9 ; the result will be the difference in feet, which 

 must be added to the elevation of the bench-mark if the barometer there was 

 higher than at the gauge, and deducted if it was lower."* 



If, however, gi-eat accuracy should be required, standard mercurial baro- 

 meters must be used, and certain corrections applied as given in Negretti and 

 Zambra's work on meteorological instruments. 



Then the work to calculate the height of a hill (Garway, in this instance) 



will be as follows : — 



Barometer at upper station ... ... ... ... 28'872 



Barometer at lower station ... ... ... ... 29-924 



Then from Table I. (see Negretti and Zambra's Treatise on Meteorological 

 Instruments, p. 42) take out the following factors : — 



28-872=1899-43 

 29-924= 923-12 



Tab. II. 976-31 



Correction for mean temperature (47 '1) 1 031 



Tab III. 1006-57561 



Con-eotion for la titude -99936 



Tab. IV. 1005-9314 &o. 



Additive correction 3- 



1008-9314 &o. 

 Lower barometer 4 ft. 10 in. above ground. 

 Upper barometer 18 in. above ground. 



1009 feet nearly 

 Add 4 10 



1013 10 

 Deduct 1 6 



1012 4 Height above Hereford. 



Mr. SymoDS supposes that the observer has only one instrument. 



