METEOROLOGY OF BEN NEVIS. xxxi 



of the unremitting care and attention required, when it is stated that in winter, 

 on occasions of extraordinary dryness of the air, it sometimes happens that the 

 cloth of the wet bulb thermometer, after being wetted and then covered with a 

 coating of ice, becomes dry again in ten minutes. It is, however, nearly always 

 found that a little attention to the wet bulb after each observation secures 

 its being in good order for next hour's observation. 



But perhaps the greatest difficulty of all is with the dry bulb thermometer. 

 Under the conditions referred to above, the dry bulb in winter gets rapidly 

 encrusted with a thin pellicle of ice, and in the warmer season with a film of 

 water with a drop ever and anon falling from it. The result is that the dry 

 bulb becomes an almost ideally perfect wet bulb ; and hence as the wet bulb is 

 less sensitive to changes of temperature, owing to the muslin surrounding it, it 

 frequently occurs that the dry bulb reads lower than the wet. In these 

 circumstances a real dry bulb is impossible to be had, but in such cases the 

 air is either at, or very near the point of, saturation. 



The hygrometric observations are printed as observed, and in the meantime, 

 while the interpretation of the wet bulb readings is often obscure and doubtful, 

 two monthly means for the wet bulb were prepared from the Tables — the first 

 being the simple means of the observations as they stand, from which Table II., 

 showing the relative humidity, was prepared ; while the second was constructed 

 on the assumption that in all cases where the dry bulb reads lower than 

 the wet bulb, the air was saturated. From the latter of these means it appears 

 that, as regards the seasons, the greatest depression of wet bulb below the 

 dry bulb occurs in summer between 11 a.m. and 6 p.m., the mean depression 

 being very nearly 1°0, and the least depression in winter from 11 p.m. to 4 a.m. 

 As the low humidities depend almost altogether on the passage over the 

 Observatory of anticyclones, or particular parts of anticyclones, which occur 

 irregularly at all seasons and at all hours, many years will be required to give 

 approximately true monthly and hourly means. The daily range of the hourly 

 means is in all cases surprisingly small, especially when the bright sunshine, 

 usually accompanying the low humidities, is taken into consideration. Even in 

 summer, the least depression is 0°70 at 3 a.m. and the greatest l o- 05 at 3 p.m., 

 the daily range being thus only o- 35, or about the third of a degree. Table II. 

 shows in the hourly means for the year an increased relative humidity from 5 

 p.m. to 3 a.m., and a diminished humidity from 4 a.m. to 4 p.m. These devia- 

 tions are very small, in every case less than one per cent. The larger deviations 

 in several of the separate months are solely due to excessively dry states of the 

 atmosphere which occurred during these months in one or more of the years. 



The immense importance of the hygrometric observations is in the decided 

 part they play in the occurrence of daily changes of weather attendant on the 

 cyclones and anticyclones of north-western Europe, a discussion of which is now 



