690 MR R. T. OMON.D ON 



Observations at the Extra Stations. 



The temperature observations made at the three additional stations will be found in 

 Table V., page 692. Mr Wedderburn and Mr Aitken were unable to make simul- 

 taneous observations at all the stations, but they arranged that the readings at A, B, 

 and C should be taken at fixed hours, and that at each of these hours a reading should 

 also be taken at the Mid-Station. At these three stations there was no instance of the 

 wet bulb reading higher than the dry, though it was occasionally equal to it, which would 

 appear to indicate that the bulbs in the Assmann psychrometer are better protected from 

 radiation than those in a Stevenson screen. 



Table VI. shows the difference of each temperature reading at these three stations 

 from the simultaneous reading at the Mid-Station. At the bottom of the table is given 

 the standard difference, that is, the difference which would exist supposing the tempera- 

 ture to decrease regularly at the rate of one degree for every 270 feet, which is the 

 average rate of change between the Base and Summit of Ben Nevis. It will be seen 

 that the observed differences vary considerably from these standard values, indicating 

 that the temperature did not decrease regularly with height at the hours covered by 

 these observations. This is a point of some interest, as these hours range from 11 a.m. 

 to 5 p.m., and do not include the night and early morning, at which times inversions 

 and vertical irregularities of temperature are most frequent. During these 14 days, 7th 

 to 20th August inclusive, the mean temperature at Ben Nevis was 37°'6 and at Fort- 

 William 53°"8, in each case computed from the mean of the 24 hourly values on each 

 day, thus giving a difference of 16° "2, which is only slightly greater than the average 

 August difference between the Summit and Base. The mean of the two is 45°"7, and 

 the mean temperature at the Mid-Station, average of daily maximum and minimum, is 

 46° *1, that is, 0°*4 higher. At the hours of observation, however, the temperatures at 

 the Mid-Station were more often below the mean of Summit and Base at the same hour 

 than above it. 



In Table VII. is given, first the mean of the Ben Nevis and Fort- William tempera- 

 tures at each hour of these observations, and second the differences of the Mid-Station 

 from these values. It will be seen that the Mid-Station ranges from about three degrees 

 above to an equal amount below the mean of Summit and Base. A comparison of this 

 table of differences with the differences of the three extra stations from the Mid-Station 

 given in Table VI. does not show any evident connection between them : the vertical 

 distribution of temperature on the slopes of Ben Nevis is a very complex phenomenon. 



In regard to the general weather of August 1902, the Meteorological Office reports 

 show that from the 7th to the 12th depressions were passing over or near to the British 

 Islands ; that on the 13th a temporary improvement began, an anti-cyclone coming up 

 from the south ; but by the 17th shallow depressions had again begun to dominate our 

 weather, and continued to do so till the 20th. There was thus no long or well-marked 

 spell of fine weather while these readings were being taken. 



