

TRANSACTIONS OF THE SECTIONS. 17 
the spectator over the flat sandy shore, and on examining the state of the locality 
where the phenomenon had been seen, the following facts were ascertained :— 
The temperature on the adjoining dry sand-hills was 87° 
on the moist sand of the flat shore 78° 1 


ee of a dry-bulb thermometer in air ... 65°°5 
of a wet-bulb thermometer in air... 63°°6 
Difference between the two last.......sseseseseeeees 1°°9 
To account for these facts, Mr. Hopkins said that when mirage appeared the sun 
was shining brightly, and by his direct rays raised the temperature of the ground 
considerably, when energetic evaporation from the wet sandy shore took place, 
which sent much vapour into the atmosphere. The presence of this vapour in the 
air checked evaporation from the wet-bulb thermometer, and prevented it from be- 
coming much cooled, and the wet-bulb thermometer at the same time, by the feeble- 
hess of its evaporation, proved the existence of the large amount of vapour in the 
locality. Now as mirage appeared only when the sun produced a large amount of 
vapour from the moist surface of the ground, which vapour was shown to be present 
by the state of the wet-bulb thermometer, it is to be inferred that the vapour caused 
the appearance of mirage. 
It might be that some of the vapour was condensed by the comparatively cool air, 
at a small distance from the surface of the sand, and thus a stratum of cloud was 
formed from the surface of which light was reflected. But however this may be, the 
presence of vapour sufficient to saturate or nearly saturate the air in the part always 
accompanied the appearance of the mirage, and therefore is presumed to be the 
cause of it. Objects that were beyond the place where the mirage appeared were 
reflected by it as if they were reflected by water. Refraction sometimes accompa- 
nies mirage, distorting the reflected as well as other objects, nearer to the spectator 
than the mirage; but the refraction is quite a separate phenomenon, sometimes 
appearing with and sometimes without the mirage. 
_ Mr. Hopkins exhibited a number of tabulated observations in corroboration of 
what had been advanced. He also said that recently, at Blackpool, in the middle 
of the day, with a clear sky and a strong sun, while the dry- and wet-bulb thermo- 
meters on the wet sandy shore were at the same height (70°), there was a difference 
of 5° between the two instruments on the adjoining cliffs, about sixteen yards high, 
These facts, Mr. Hopkins contended, proved, that while evaporation saturated the 
air near the surface and produced mirage, the atmosphere at the height named was 
comparatively dry, allowing evaporation to take place with considerable energy from 
the wet-bulb thermometer. 
Letter from Sir Rozert H. Ineuis, Bart., F.R.S., to Col. Sainz, R.A., 
’ Aug. 8, 1849. 
We were at Gais (Canton Appenzell, Switzerland) a few days ago, and saw 
there, what may be familiar to you and other men of science, but was quite new to 
me and to the people at the place. About 3 p.m. on the 8th of August my servant 
called to me, ‘‘ that there was something falling very curious.”” I went out—to the 
pidge which connects the old and new buildings of the Hotel du Boeuf,—and under 
the shade of the new house looked up and saw thousands and thousands of brilliant 
white motes, like snow, falling as in flakes. There were no clouds, but there was 
a kind of halo round the sun, or rather, as I looked up, there were in that direction 
need more and larger masses through which the rays passed; balls separated 
t emselves, consisting of vast numbers, and these resolved themselves into frag- 
ments and came whirling and floating about. ‘The master of the Hotel, M. Heen, 
joined us: he had obviously never seen anything of the kind before, and called out, 
* Des millions, des millions.” He summoned his people to look. I continued to 
gaze till I was half-blinded. At first the fragments seemed to melt; and to the 
last I could distinguish no appearance of an animal. Our servant fancied that he 
Saw something like wings; I certainly looked till, to my eye, they seemed to evapo- 
Tate, but their disappearance and perhaps the re-appearance of the same individual, 
might have been owing to theit turning at right angles instead of exhibiting their 
sar lengthways, and vice versd. This lasted—at least Ilooked—25 minutes. Cer- 
9. 2 
