1886.] Mr R. T. Omond on Temperatures at Ben Nevis. 27 
at only two hours do they exceed one-tenth of a degree. This 
small difference on the foggy days fully bears out what I have 
formerly observed on Ben Nevis, that in a saturated atmosphere — 
with mist or fog present, it makes no practical difference how the 
thermometers are placed, so long as the air can reach them at all 
and they are shaded from the direct rays of the sun. 
Differences of High from Loiv Thermometers. 
As the surface of the hill top consists entirely of broken rock 
without soil or vegetation, it seems probable that a large amount 
of the difference between the two thermometers is directly due to 
ground radiation. On a calm day with bright sunshine the stones 
get so heated as to be disagreeable to handle, but there were un- 
fortunately no such days last August ; still, the radiation of heat 
during the day and of cold at night during ordinary clear weather 
must be considerable, and the lower thermometer was much nearer 
this source of heat and cold than the upper. It should, however, 
be borne in mind that, as the screens are only open below and the 
bulbs of the thermometers raised about four inches above the lower 
