638 M.E JOHN AITKEN ON THE NUMBER OF DUST PARTICLES IN THE 



We have here, as in Part II., looked upon the daily maximum on the Bigi as produced 

 by the rising of the valley air to the Kulm under the influence of winds and sunshine. 

 When we come to consider the Kingairloch observations we shall find that this conclusion 

 may require some modification. There seems reason for supposing that the above explana- 

 tion does not contain the whole truth, and, though it may explain the greater part of 

 the increase in the numbers, there yet appears to be reason for believing other influences 

 to be at work in producing the result. 



Cloudy Pilatus. 



Though the cloudiness of Pilatus does not bear directly on our present investigation, 

 yet, as it has been constantly under observation during the visits to the Rigi, a few 

 remarks on the subject may not be entirely out of place here. The tendency of Pilatus 

 to be shrouded in clouds has for long been a well-recognised feature of the mountain, and 

 has given rise to its present name (Mons Pileatus, the capped mountain). While observing 

 on the Rigi, this tendency of Pilatus to cloud over was frequently noticed. On many 

 days when the Rigi was clear of cloud Pilatus was covered. This we are entitled to expect 

 to take place to a certain extent, as the mountain is 1000 feet higher than the Rigi. But 

 the clouds on Pilatus frequently descended its slopes to far below the level of the Rigi, 

 while there was no tendency for clouds to form on the Rigi. It is this freeness from 

 clouds on the Rigi which has made me keep to it as an observing station, though it is 

 lower than Pilatus. 



However fascinating the legends may be that are given to account for the seeming 

 gloomy and morose humours of Pilatus, yet one cannot view the different behaviour of the 

 two mountains without looking for some physical explanation of it. In many respects the 

 two mountains look similarly situated. The Rigi is certainly an isolated mountain, and 

 Pilatus, according to the guide-books, is " almost isolated from the surrounding heights," 

 and it must be admitted that this mountain, as generally seen by visitors, looks quite as 

 isolated as the Rigi. This, however, is very far from being the case. The Rigi is a true 

 isolated mountain, with valleys on all sides, and none of these valleys rises much above 

 the level of the Lake of Lucerne ; while Pilatus is only a terminal peak of a long and 

 mighty wall of mountains. As seen from Lucerne and many other points, Pilatus looks 

 as if it were isolated, but if we view it from the north, we shall see it is the eastmost 

 peak of a grand range of mountains, which extends westwards in an unbroken line for 

 about 25 miles. An examination of a relief chart of Switzerland shows that this range 

 forms a fairly regular wall, which starts from Pilatus in a westerly direction, and then 

 turns slightly southwards. The upper ridges of these mountains are in many places fairly 

 horizontal, and rise to an elevation of over G000 feet, while about the middle of the range 

 they are 7000 feet. It is not now difficult to understand why Pilatus is so much more 

 cloudy than the Rigi. The Rigi being an isolated mountain, there is but little tendency 

 for the valley air to be driven up its slopes by the wind ; but when the wind blows 

 against the great wall of which Pilatus is the terminal peak, it is forced to ascend, and on 



