xl METEOROLOGY OF BEN NEVIS. 



irregular hollow bodies bristling all over with pointed crystals, and the arms are 

 increased to many times their original thickness, and ultimately the whole instru- 

 ment becomes an irregular mass of immovable snow, and of course no observation 

 with it is possible. If the formation of the crystals begin while the anemometer 

 is still turning, the mode of formation is interesting, as throwing light on the 

 behaviour of the wind at different points of the instruments. The crystals form 

 mostly on the outside of the cups and round their edges, leaving the insides pretty 

 clear; the arms carrying the cups get completely covered; and on the diagonal 

 stays supporting the arms the crystals show a beautiful twined structure, pointing 

 downwards and outwards on each side. 



But the most serious consequences are those which affect the thermometers. 

 The louvres of the Stevenson's screen for the thermometers become serrated with 

 rows of teeth, which quickly coalesce into a solid mass, completely preventing the 

 possibility of any circulation of the air inside the box. In short, these instruments 

 are no longer in contact with the free atmosphere, and unless a fresh box is fixed 

 up by the observers no observation of the outside temperature of the air would be 

 possible. As it is impossible to devise any arrangement by which the instrument 

 for the temperature of the air could be kept clear of these crystals, or by which it 

 would be secured that it would be constantly exposed in the free atmosphere, the 

 temperature can never be recorded by self-registering instruments. And since 

 barometric observations are but of small value unless accompanied with simul- 

 taneous observations of the temperature of the air, it follows that the services of a 

 staff of observers for hourly observations of temperature by night as well as by day 

 can never be dispensed with. 4 '" 



Winds and Rainfall of Ben Nevis. — Mr Omond has examined this subject 

 minutely from the observations of 1885 and 1886. From this examination it was 

 found that the order of the frequency of the winds on the top of the mountain was, 

 with the percentages of frequency, N. 15; S.E. 14; S.W. 14; W. 13; S. 12; N.E. 11; 

 N.W. 9; E. 7; and calms, 5. In each of the two years, E. and N.W. winds showed 

 the least degree of frequency. 



For the two years 1885 and 1886 the winds, arranged in the order of their 

 wetness, with the percentage of the whole rainfall that fell with each wind, were 

 W. 21; S.W. 20; N.W. 18; N. 13; S. 8; N.E. 7; S.E. 5; E. 3; and calms, 5. Thus 

 with N., N.W., W. and S.W. winds, 72 per cent, of the whole falls, whilst for the 

 other half of the compass only 23 per cent, falls. Hence the westerly winds are by 

 far the rainiest, and easterly winds the driest in 1886. The year was divided nearly 

 equally into cyclonic and non-cyclonic periods, there being 4335 hours of the 

 former and 4425 hours of the latter. Nearly four-fifths of the whole rainfall fell 

 during cyclonic periods. In cyclonic periods the percentage of precipitation with 



* " On the Formation of Snow Crystals from Fog," by R. T. Omond, Jour. Scot. Met. Soc, Third Series, 

 vol. i. p. 190. 



