METEOROLOGY OF BEN NEVIS IN CLEAR AND IN EOGGY WEATHER. 781 



stance of water in its changes of state of aggregation. It cannot be doubted that, 

 considering the Earth as a whole, the nett effect of these secondary agencies is nil. 

 Yet their local effect is in many cases very great, as witness the difference in population 

 between Great Britain and places in the same latitude in North America. The waters 

 of the ocean are also a powerful secondary agency in the distribution of the sun's 

 heat. 



The 3^ years' published observations on Ben Nevis, though furnishing excellent 

 material for the illustration of lectures, were not sufficient for the thorough discussion 

 of the climate of the mountain. There was no lack of foggy weather ; but it is of very 

 little use attempting to study a secondary effect without knowing something of the 

 primary effect which is disturbed, and we must consider the fog as a disturbance of the 

 normal solar climate. For this purpose it is important to have a sufficient number 

 of examples of the clear weather of Ben Nevis ; and now that ten more years have 

 passed, thirteen whole years' observations are available, and these have been obligingly 

 put at my disposal by Dr Buchan. 



Many special features of the meteorology of Ben Nevis have been discussed in 

 interesting papers by Mr Omond, Mr Rankine, Mr Mossman, and by Dr Buchan. 

 I have little doubt that these papers would have gained in interest if the clear and 

 foggy weather had been distinguished. It has been my wish to do something towards 

 this end myself, and at the beginning of last winter I set about sorting the thirteen 

 years into periods of clear and of foggy weather, leaving out all days of mixed weather. 

 The burden of this work has been borne by Mr Andrew King, who for a number of 

 years ably assisted me in my chemical work. Without his aid the collection of books, 

 with the hourly observations sorted into spells of clear and of foggy weather, would 

 never have been brought together ; and I desire to express my great obligation to him 

 for this and for the extensive work of computation which has accompanied it. 



Great as this labour was, it would have been several times greater had our 

 meteorological observatories followed the fashion, which it is attempted to force on 

 them from abroad, more especially from the continent of Europe, of using Celsius' 

 scale for the measurement of the temperature of the air. * 



The books, which already form almost a little library, will be deposited with the 

 Scottish Meteorological Society, and will furnish the indispensable preliminary arrange- 

 ment for permitting every feature of the meteorology of Ben Nevis to be discussed with 

 reference to clear or foggy weather. I attach particular value to discussions by men 

 like Mr Omond, Mr Rankine, Mr Bruce, and others who have resided on the summit 

 for a considerable time as observers. They have known the mountain in all its moods, 

 and explanations will occur to them which would never be suggested to another by the 

 mere contemplation of numbers. 



The published observations were begun on Ben Nevis in 1884, but, to avoid a 

 broken series, I have taken the years from 1st January 1885 to 31st December 1897. 



* My views on this subject are more fully explained in a letter which appeared in Nature (1899), vol. lx. p. 364. 



