THE MID-STATION ON BEN NEVIS 689 



the greater fall taking place in the half-day ending at 9 a.m. The third column 

 gives the total for the 24 hours ending at 9 a.m., entered as usual to the previous 

 day. During this time, 7th to 29th August, the rainfall at the Summit was 6 '32 inches, 

 and at Fort- William 3 '81 inches. 



Table IV. — The temperature, dry and wet bulbs, and self- registering maximum 

 and minimum thermometers read and set at 22 h in the Stevenson screen. There were 

 many cases of the wet bulb reading higher than the dry, and attention is drawn to them 

 in this table by printing the wet-bulb entry in italics in all such cases. A comparison 

 with the cloud table shows that in every instance except one (11th August, at 22 h ) there 

 was either mist, fog, or overcast sky when this occurred. The dry bulb under these 

 circumstances is dripping with moisture, and the higher reading of the wet bulb is 

 sometimes explained by the supposition that the dry bulb is acting as a more perfect 

 wet bulb than the muslin-covered one. But it is difficult to see how any evaporation, 

 and consequent cooling, can be taking place from either when moisture is being deposited 

 on the dry bulb. It may be that vapour is being condensed into water on each, but 

 more rapidly on the muslin-covered wet bulb, which is therefore raised in temperature 

 more than the dry bulb by the consequent liberation of heat, but more probably the 

 higher temperature of the wet' bulb is a direct effect of radiation. The screen does not 

 perfectly protect the bulbs from solar radiation, especially not from reflected radiation 

 from the ground, and the rougher surface of the muslin-covered bulb is doubtless a 

 better absorber of radiant heat than the wet glass of the dry bulb. Either of the two 

 latter explanations indicates that the temperatures recorded by both bulbs are higher 

 than the true air temperature, while the other explanation, that the dry bulb acts as 

 the more perfect wet bulb of the two, implies that both are below the true air tempera- 

 ture. This curious case of the wet bulb being a few tenths of a degree above the dry 

 has frequently been observed during the day time at the Summit, and it is often an 

 indication there that a fog is going soon to disperse. It has also been noted that at 

 night sometimes the wet may be a little below the dry, though the air is foggy and 

 saturated : an effect probably of terrestrial radiation lowering the temperature of both, 

 but the wet more than the dry. 



It is possible that during a rapid fall of temperature in a saturated or nearly 

 saturated atmosphere the wet bulb may occasionally read higher than the dry, because 

 the muslin covering prevents the change of temperature affecting the wet bulb as 

 rapidly as it does the bare dry bulb, and thus, wdien the temperature of each is falling 

 quickly, the wet lags behind the dry bulb. This, however, cannot be the cause of the 

 higher readings of the wet bulb at the Mid-Station and at the summit of Ben Nevis, 

 for at both places during such times the temperature, as a rule, is very steady, changing 

 little from hour to hour ; radiation seems the most probable explanation of this curious 

 anomaly. 



