METEOROLOGY OF BEN NEVIS. XXV 



5*106 in January, the difference being 0*410 inch. Comparing the pressures 

 at the Observatory with those at sea-level at Fort- William, the mean annual 

 difference is 4*562 inches, the greatest monthly difference being 4*626 inches 

 in March, and the least 4*483 inches in July. 



Among the more important points to be determined, particularly as regards 

 the investigation of weather phenomena, are the normal differences between 

 atmospheric pressure at the Observatory and at Fort- William for the different 

 air temperatures and sea-level pressures that occur. These differences, or 

 corrections for height as they are usually called, were first calculated from all 

 the synchronous observations made at the two stations, and the results entered 

 in a provisional " Table of Corrections for Height." With the aid of this table 

 the departures from the normals were ascertained for each of the five daily 

 observations made since the opening of the Observatory. The results showed a 

 diminution of pressure at the Observatory on almost every occasion during the 

 prevalence of high winds on the top of the mountain ; or when the wind blew 

 at a rate of 30 miles an hour and upwards, the observations reduced to 

 sea-level indicated a less pressure than what was actually observed at Fort- 

 William at the time. Further, the differences were found to increase with 

 the strength of the wind, and amounted not unfrequently to the tenth of an 

 inch, and on one day when the wind continued to blow at the rate of about 

 120 miles an hour, the five synchronous observations of that clay showed 

 differences of about 0*150 inch. This lowering of pressure is no doubt due to 

 the winds partially sucking out the air from the interior of the building as 

 they brush past the Observatory. A similar result was also observed by Mr 

 Wragge and assistants during the summer months of 1881-83, when the baro- 

 meter was placed in a cairn specially built for its reception. 



It thus became necessary to calculate afresh the table of corrections for 

 height, using in the new calculation only those observations which were made 

 when the wind at the Observatory blew at rates lower than 30 miles an hour. 

 This recalculation has been completed, and with the results thus obtained, the 

 " Table of Corrections for Height " accompanying this paper has been prepared, 

 giving the corrections for each tenth of an inch of the pressure at sea-level 

 from 27*400 inches to 30*900 inches, and for every 1°*0 of the temperature of 

 the air from 16°*0 to 75° 0. In this inquiry the temperature of the air has 

 been assumed as the simple arithmetical mean of the temperatures at the 

 Observatory and Fort- William. 



The results are closely accordant with the figures for a sea-level pressure of 

 30*000 inches published by General Hazen of the U.S. Signal Service for 

 the use of the United States' observers, with this difference, that for each 

 degree from 16°*0 to 75°*0, the sea-level corrections empirically determined for 

 Ben Nevis are 0*020 inch greater than the figures of the Signal Office, 

 vol. xxxiv. d 



