286 



REPORT 1893. 



is low. Owing to the bigh winds wliicli often prevail at the time, and 

 the formation of the fog being chiefly confiueil to the restricted area of 

 the mountain top, the increased pressure is not relieved by the formation 

 of an asoeudiug current, and hence pressure is increased at the top, being 

 the restricted area where the condensation takes place. 



Table V. — Showing Difference of Pressure, in Thousandths of an Inch, at 

 the Ben Nevis Observatory, 4,406 feet, between foggy days and clear 

 days respectively. The plus sign shows jiressure on foggy days the 

 greater ; the minus sign, less. 



The work of recopying on daily sheets the hourly observations of 

 both observatories, which show at a glance the relations of the two sets 

 of observations to each other, is in progress, and already about half of the 

 three yeai's is finished. These sheets show the relative times of occur- 

 rence at the top and bottom of the mountain respectively of changes of 

 pressure, temperature, humidity, and the other subjects of observation, 

 together with their relative amounts. As the work proceeds the entries on 

 the sheets are compared with the bi-diurnal weather maps of the weekly 

 weather report of the Meteorological Office, and copious notes are made 

 of the relations of the observations of the two observatories to the cyclones 

 and anticyclones of north-western Europe at the time. One example may 

 be here referred to. It frequently happens that the temperature difier- 

 ence of the two observatories, which is generally about 16°"0, becomes less 

 and less during the time of the anticyclone, and occasionally temperatui'e 

 is higher at the top than at the foot of the mountain. But as the anti- 

 cyclone gives way, and the cyclone advances, temperatures assume their 

 normal difference. Now the observations show two marked types of 

 weather in these circumstances : one typ4 when the difference is brought 

 about by a falling temperature at the top while temperature at Fort 

 William remains practically stationary ; and the other type when tempera- 

 ture rises at Fort William while at the top of Ben Nevis it is stationary. 

 These different types at the present stage of the inquiry seem to point to 

 important well-marked characteristics of the approaching weather. 



