134 ~° ° Altitudes of Mountains, Sc. visible from 
be remarked that the nearer the ray passes to the ground, 
the greater the variation. In general, the refraction when vari- 
able is greatest near sun-rise and sun-set, and least during the 
heat of the day. Even when the diurnal variation is scarcely 
perceptible, a very sudden increase of refraction of 10" or 20” 
will be remarked on an evening within a short time of sun-set. 
I have had no opportunities of determining whether the refrac- 
tion remains at its maximum or not during the night. 
At none. of the other stations could any certain proofs of the 
existence of this diurnal refraction be established; but then it 
must be recollected, that its effects had already ceased at Rum- 
bles Moor, and most probably at the observatory. .It might not 
have heen witnessed at Beamsley Rock on account of the sides 
of the mountain being excessively steep in almost every direction, 
and perhaps from the extreme dryness of the surface. 
In attempting to account for this peculiar refraction, it was 
in the first place conjectured that the superior strata of the at- 
mosphere might be heated at sun-rise and sun-set in a greater 
degree than the inferior ones. 
Many observations made with the thermometer ‘at the base 
ard summit of Rumbles Moor, tended rather to refute than to 
confirm this hypothesis. It could scarcely be occasioned by the 
morning and evening frosts, or its effects would have been again 
perceptible in the autumn. There is little doubt, however, that 
the stratum of air immediately in contact with the surface of the 
ground is hotter about noon, and undoubtedly colder at morning 
and evening than the succeeding one; but why this irregularity 
should be confined to five months in the year is not quite so ex- 
plicable. It serves however to account for the non-existence of 
the variation on a steep craggy mountain, such as Beamsley 
Rock. 
Observers have generally remarked, at one time or other, cases 
of sudden and extraordinary refraction ; but the following is the 
only marked one that has come under my notice :—February 9, 
1821, the moor appeared under an angle ‘of elevation of 42’ 30” 
at 175 15™; yet in the course of a quarter of an hour it was 
found iedncaced to 43’ 18”, The thermometer, which was then 
at 41°, fell very rapidly, and shortly after rose as abruptly. 
The sun’s vertical diameter was unusually contracted, and its 
contour curiously indented; at 10° of the same day the ther- 
mometer was at 41 on the moor (then invisible) and only at 35 
at the observatory, 
The following observations will (with one exception) serve to 
verify the theory of the refraction being affected by an wnusual 
difference of temperature at the two stations, and will also ren- 
ier the diurnal variation more intelligible. They point out, 
moreover, 
