CHANGE OF TEMPERATURE WITH HEIGHT DURING ANTI-CYCLONES. 507 



The Fort-William barometer at 32° and sea-level was 



The Fort- William temperature was 



The Ren Nevis temperature was .... 



Giving an average temperature of . 



The Ren Nevis barometer at 32° was 



The entry in table for a sea-Level pressure of 29'909 and a mean temperature of 3S"0 i 



Which added to Ben Nevis barometer gives ....... 



29-909 

 46-3 

 29-8 

 38-0 

 25-343 

 4-567 

 29-910 



The last entry is, therefore, the Ben Nevis barometer reduced to sea-level, and it 

 agrees closely with the actual sea-level pressure as indicated by the Fort-William 

 barometer. But this agreement does not always take place, and, as stated above, 

 during anti-cyclones they seldom agree, the Ben Nevis barometer frequently coming 

 out higher than the true sea-level pressure. For example, on 18th February 1893, at 

 10 p.m. : — 



The Fort- William barometer at 32° and sea-level was 



The Fort- William temperature was .... 



The Ben Nevis temperature was .... 



Giving an average temperature of 



The Ben Nevis barometer at 32° was 



The entry in table for a sea-level pressure of 29'700 and a mean temperature of 35*1 is 



Which added to Ben Nevis barometer gives ........ 



29-700 

 38-0 

 32-2 

 35-1 

 25-177 

 4-558 

 29-735 





This last entry — the Ben Nevis barometer at sea-level — differs from the true sea-level 

 pressure recorded at Fort- William by + 0'035 inch, and differences like this occur whenever, 

 during anti-cyclones, the Ben Nevis temperature rises near to or surpasses that at Fort- 

 William ; while during cyclonic weather, the Ben Nevis barometer tends to come out 

 lower than the true sea-level pressure. These differences frequently amount to 0*040 

 or 0*050, which is about 1 per cent, of the whole difference of pressure between summit 

 and base ; they never in anti- cyclones amount to O'lOO. 



The most obvious way for accounting for a difference such as we get in the second 

 example is to assume that the wrong temperature column has been taken in the reduc- 

 tion table, that is, that the mean of the Ben Nevis and Fort- William temperatures is 

 not the true average for the air between these levels. If we assume an average 

 temperature of 39° '6 instead of 35 c "l as given above, the " Entry in Table" becomes 

 4 "523 instead of 4*558, and the barometers agree. If this hypothesis — that the 

 difference of the barometers is due to a faulty method of estimating the temperature of 

 the air between the two stations — be true, then it follows that when the Ben Nevis 

 barometer comes out too high, the intermediate air is warmer than the mean of the 

 Ben Nevis and Fort- William temperatures by an amount proportional to the sea-level 

 difference of the two barometers. 



To test the truth of this hypothesis, observations are needed at heights intermediate 

 between the summit and base. Of such the only data yet collected at Ben Nevis are 

 (first) Mr Wragge's observations, made by himself and his assistants when ascending 

 and descending the mountain, and (second) a valuable series made by Mr Muir at a 

 temporary half-way station in September 1896. The former are not of much use for 



