518 



MR OMOND ON THE BEN NEVIS AND FORT-WILLIAM BAROMETERS 



way, are wholly due to a faulty determination of the temperature of this air. The first 

 assumption seems reasonable, and the second is borne out by the fact mentioned above, 

 that the differences of the barometer seem to depend on a radiation effect. The 

 Departure tables show that these computed temperatures have a less daily range than 

 the observed temperatures, and that the diurnal changes are all delayed for two or three 

 hours. A mass of free air, not in immediate contact with the ground, is likely to differ 

 in temperature just in those ways from air near the ground, such as that whose tempera- 

 ture is taken in the screens at the Ben Nevis and Fort- William Observatories ; and this 

 last table gives us at least an approximation to the mean temperature of the whole 

 mass of air up to 4400 feet in height. There may be other factors which, influencing the 

 diurnal barometric values differently at the two Observatories, would slightly modify 

 the numerical values of these computed temperatures, but I believe the Departure table 

 shows substantially the hour-to-hour changes of temperature in the free air above Fort- 

 William up to 4400 feet. 



The difference between the mean of the observed temperatures at Ben Nevis and 

 Fort- William and the temperature of the intermediate air, as computed from the baro- 

 metric readings at the two Observatories, is given for each hour in the small table 

 below. The curves for each season in this table are clearly radiation effects, symmet- 

 rical about the times at which solar and terrestrial radiation are respectively most 

 powerful, and not about the times when the temperature of the air is highest and 

 lowest. The effect is greatest in spring and summer, that is from March to August, 

 when the sun is most powerful, and least in winter, that is from December to February, 

 when solar action is weak, and when the Fort-William thermometers are shaded in the 

 forenoon by the hills that rise to the south-east. 



TABLE III. — Difference at each Hour between the observed Temperature (Mean of Ben Nevis 

 and Fort- William Temperatures), and the Temperature computed from the Barometric Readings. 



When the observed is the higher, the difference is put in bold type, and when it is the lower 



in italic type. 





1 



2 



3 



4 



5 



6 



7 



8 



9 



10 



11 



12 



13 



14 



15 



16 



17 



18 



19 



20 



21 



22 



23 



Mid- 

 night. 



Spring . 



i-o 



1-8 



1-8 



1-6 



IS 



■9 



1 



1-0 



17 



21 



2-4 



2-4 



21 



17 



17 



1-5 



•8 



•4 



•5 



■8 



IS 



1-5 



16 



1-7 



Summer . 



19 



1-8 



1:9 1-8 



1-4 



■5 



•2 



1-0 



1-5 



18 



18 



1*6 



1-5 



1-6 



16 



1-2 



•9 



7 



■o 



•8 



V3 



1-6 



1-8 



1-8 



Autumn . 



V2 



V.! 



1-3 



1-1 



V2 



1-3 



V3 



'5 



•4 



•8 



15 



1'8 



1-9 



2-0 



1-9 



1-5 



•5 



•2 



■5 



■7 



■9 



l'O 



1-0 



•9 



Winter . 



■5 



•5 



'4 



■5 



•6 



■3 



■5 



•5 



•3 



1 



•5 



10 



1-3 



1'4 



11 



•6 



1 



'2 



•3 



■2 



•4 



'3 



■2 



•1 



Year 



1-4 



1-3 



1-4 



1-3 



1-2 



•8 



•4 



•3 



•8 1-2 



1-6 



17 



17 



17 



1-6 



1'2 



•6 



•3 



■3 



■6 



1-0 



1-1 



11 



1-1 



