INTER-RELATIONS OF BAROMETRIC PRESSURE. 



513 



August, these being respectively 367 and 403 days; and the smallest in January and 

 February, these being 261 and 259 days. 



The tabulations had not proceeded far when it became evident that the most im- 

 portant point to be kept in view in investigating the bearing of the Ben Nevis observa- 

 tions on weather changes was the difference between the temperatures simultaneously 

 recorded at the two Observatories. The result was the tabulation of the observations of 

 Table T. into five classes, viz. : — 



Class I. — Mean Monthly Sea-level Pressures at Fort- William in inches. 



Class II. — Mean Monthly Temperature at both Observatories. 



Class III. — Mean daily Sunshine in hours at both Observatories. 



Class IV. — Mean daily Rainfall in inches at both Observatories. 



Class V. — Mean Depression of Wet Bulb at both Observatories. 



Each of these five classes is again divided into four groups, in accordance with the 

 observed temperature : — 



Group I. — Results for all temperature differences observed. 



Group II. — Results for temperature differences from 13°*0 to 17°*0, which, being 

 substantially the same as those for Group I, need not here be considered. 

 Group III. — Results for temperature differences under 12 o, 0, and 

 Group IV. — Results for temperature differences above 18°*0. 



The results of the above will now be given as a necessary preliminary to Tables II. 

 and III. 



Class I. — The Mean Monthly Sea-Level Pressure at Fort-William. 





Jan. 



Feb. 



Mar. 



Apr. 



May. 



June. 



July. 



Aug. 



Sept. 



Oct. 



Nov. 



Dec. 



Year. 



Group I. — 



Ins. 



Ins. 



Ins. 



Ins. 



Ins. 



Ins. 



Ins. 



Ins. 



Ins. 



Ins. 



Ins. 



Ins. 



Ins. 



Temperature Differences, 

 all observed 



29-863 



29-845 



29-814 



29-894 



29-969 



29-963 



29-916 



29-835 



29-879 



29*747 



29718 



29-625 



29-839 



Group III. — 



Temperature Differences, 

 12°"0 or less 



30-171 



30-115 



30-158 



30-167 



30"206 



30-165 



30-070 



30-200 



30-198 



30-173 



30-231 



30-003 



30-157 



Group IV.— 



Temperature Differences, 





























18° '0 or greater . 



29-843 



29791 



29-824 



29-901 



29-707 



29-816 



29-903 



29-831 



29-832 



29-775 



29-785 



29-850 



29-837 



Group III. — 





























Excess above Group I. 



•308 



•270 



•343 



•273 



•237 



•202 



•154 



•365 



■319 



•426 



•513 



•378 



•318 



Group III.— 



Excess above Group IV. 



•328 



•324 



•334 



•266 



•299 



•349 



•167 



•369 



•366 



•398 



•446 



•153 



•320 



Thus on the days when the differences between the mean daily temperatures were 

 small, 12°*0 or less, the mean barometric pressure at Fort- William was unwontedly high 

 for each of the months of the year, the excess above Group I. amounting on the mean of 

 the year to 0*318 inch. The mean excess from October to March is 0*365 inch, and for 

 the six months from April to September 0*257 inch. The greatest excess was 0*513 inch 

 for November, and the least 0*154 inch for July. 



TRANS. ROY. S0C. EDIN. — VOL. XLIII. 3 T 



