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The Reduction of the Ben Nevis Barometer to Sea-level. 



A table for reducing the barometric readings at Ben Nevis Observatory to sea-level 

 was given in the former volume of observations, and is reprinted here. The table was 

 compiled by direct comparison of the barometric readings at the Ben Nevis Observatory 

 and at the Public School, Fort- William, and consists of the average difference between 

 these barometers, both being reduced to 32°, and the Fort- William one brought to sea- 

 level, for each tenth of an inch of pressure from 30*9 to 27 '4 inches, and for each 

 degree of temperature from 16° to 75°. Any missing values were filled in by interpola- 

 tion, and the whole smoothed by cross averaging. A full description of the preparation 

 of the table, which is regarded as of a provisional character, will be found in the former 

 volume. (See Transactions Royal Society, Edinburgh, vol. xxxiv. p. xxv.) 



The table gives the corrections to be added to the Ben Nevis barometer to 

 reduce it to sea-level, arranged for each tenth of an inch of pressure and each 

 degree of temperature, and an interpolation table is appended for the intermediate 

 thousandths of an inch and tenths of a degree. The differences between the successive 



o 



lines and columns of the main table are so uniform that the same interpolations may 

 be used for all parts of the table without introducing an error of more than one 

 thousandth of an inch at any part. At the top of the main table are given the 

 temperatures, and at the side the successive tenths of an inch of pressure, these 

 temperatures being the mean of the Ben Nevis and Fort- William temperatures, and 

 these pressures the Fort- William barometer readings at 32° and sea-level. In the 

 interpolation table the intermediate thousandths of an inch are given at the top, and 

 the tenths of a degree at the sides ; in the body of this table are the amounts to 

 be added to or subtracted from the entries in the main table. 



Example — 9th June 1891, at Midnight. 

 Fort-AVilliani barometer reduced to 32° and sea-level, 

 Ben Nevis barometer reduced to 32°. 



Fort-William temperature, 



Ben Nevis temperature, 



Mean temperature, . 

 Correction in table for 29'9 inches and 

 Interpolation for 83 and '5, 



Correction for 29"983 and 44°-5, 





29-983 





25-451 



49°-0 





40°-0 





44°-5 

 44 degrees, . . . 4 - 521 

 + 9 



4-530 4-530 



Ben Nevis barometer reduced to seadevel, ...... 29-981 



The table of corrections extends from 4'902 inches to 4'176 inches. The fractional 

 parts of an inch only are given in each column ; it is unnecessary to repeat the 4* inches, 

 which is placed at the top and is common to the whole table. When the table was 

 constructed the only instances of very low pressures that had occurred were accompanied 



