MIR 
fame Illufion began in regard to the next village. M. 
Monge afcribes this effedt to a diminution of the denfity 
of the lower ftratum of the atmofphere. This diminu¬ 
tion in the deleft is produced by the increafe of heat, 
a fifing from that communicated by the rays of the fun to 
the land, with which this ftratum is in immediate contaft. 
At fea it takes place when, by particular circumftances, 
fuch as the adtion of the wind, the lower ftratum of the 
atmofphere holds in folution a greater quantity of water 
than the other ftrata. In this ftate of things the rays of 
light, which come from the lower parts of the heavens, 
having arrived at the furface that feparates the lefs denfe 
ftratum from thole above it, do not pals through that 
ftratum, but are reflected, and paint in the eye of the ob- 
ferver an image of the heavens, which appearing to him to 
be below the horizon, he takes it for water, when the phe¬ 
nomenon occurs at land. If he is at fea, he thinks he fees 
in the heavens all the objedls which float on that part of 
the furface occupied by the image of the heavens. 
This phenomenon has been confidered and explained 
by feveral Englifli philofophers; but moft particularly 
and minutely by Mr. Profeffor Vince, who made it the 
fubjedt of the Bakerian Ledture in 1798, the fubftance of 
which we fhall here prefent to the reader. 
“ The uncertainty of the refradtion of the air near the 
horizon has long been known to aftronomers, the mean 
refradtion varying by quantities which cannot be ac¬ 
counted for from the variations of the barometer and 
thermometer; on which account, altitudes of the hea¬ 
venly bodies, which are not more than 5 0 or 6°, ought 
never to be made ufe of when any confequences are to be 
deduced from them. The caufe of this uncertainty is 
probably the great quantities of grofs vapours, and exha¬ 
lations of various kinds, which are fufpended in the air 
near the earth's furface, and the variations to which they 
are fubjedt; caufes of which we have no inftruments to 
meafure the effedts they produce, in refradting the rays of 
light. In general, the courfe of a ray palling through the 
atmofphere, is that of a curve which is concave towards 
the earth, the eft'edt of which is to give an apparent ele¬ 
vation to the objedt; and thus the heavenly bodies appear 
above the horizon when they are adtually below it; but 
it will not alter the pofition of their parts, in refpedt to 
the horizon ; that is, the image of the higheft part of the 
object will be uppermoft, and the image of the lowed part 
will be undermoll. The figures however of the fun and 
moon, when near the horizon, will fuffer a change, in con- 
fequence of the refradtion of the under limb being greater 
than that of the upper; from which they afi'ume an ellip¬ 
tical form, the minor axis of which is perpendicular to the 
horizon, and the major axis parallel to it. But a perpen¬ 
dicular objedt, fituated on the furface of the earth, will 
not have its length altered by refradtion, the refradtion of 
the bottom being the fame as that of the top. Thefe are 
the effedts which are produced on bodies at or near the ho¬ 
rizon, in the common ftate of the atmofphere, by what I 
{ball call the ufnal refraction. 
“ But, befides the ufual refradtion which aftedts the 
rays of light, the atmofphere over the fea is fometimes 
found to be in a ftate which refradts the rays in fuch a 
manner as to produce other images of the objedt,- which 
we wall call an effect from an unufual refraction. In the 
Phil. Tranf, for 1797, Mr. Huddart has delcribed fome 
effedts of this kind, which he has accounted for by fup- 
pofing that, from the evaporation of the water, the refrac¬ 
tive power of the air is not greateft at the furface of the 
fea, but at fome diftance above it; and this will folve, in 
a very fatisfadtory manner, all the phenomena which he 
has obferved. But effects very different from thofe which 
have been deferibed by Mr. Huddart are fometimes found 
to take place. Thefe I had an opportunity of obferving 
at Ramfgate laft f'ummer, on the iftof Augult, from about 
half an hour after four o’clock in the afternoon till be¬ 
tween feven and eight. The day had been extremely hot, 
and the evening was very fultry ; the fky was clear, with 
AGE. 559 
a few flying clouds. I fhall deferibe the phenomena as I 
obferved them with a terreftrial telefcope, which magni¬ 
fied between thirty and forty times; they were vifible 
however to the naked eye. The height of the eye above 
the furface of the water, at which moft of the obfervations 
were made, was about twenty-five feet; fome of them 
however were made at about eighty feet from the furface; 
and it did not appear that any of the phenomena were 
altered by varying the height of the eye, the general ef¬ 
fedt remaining the fame. 
“ The fir ft unufual appearance obferved, was that which 
is reprefented in the preceding Plate, at fig. 2. Diredting 
my telefcope at random, to examine any objedts which 
might happen to be in view', I faw the top of the malts 
pf a fhip A, above the horizon, a-;/, of the fea, as fliowa 
in the figure; at the fame time, aifo, I difeovered in the 
field of view, two complete images, B, C, of the fhip in 
the air, vertical to the fhip itfelf, B being inverted, and 
C eredt, having their hulls joined. The phenomenon 
was fo ftrange, that I requefted a perfon prefent to look 
into the telefcope, and examine what was to be feen in it, 
who immediately deferibed the two images, as obferved 
by myfelf; indeed they were fo perfedt, that it was im- 
poflible we could differ in our defeription. On this I im¬ 
mediately took a drawing of the relative magnitudes and 
diftances of the fhip and its images, which at that time 
were as reprefented in the figure, as near as it was poffxbla 
for the eye to judge ; and it was very eafy to eftimate 
them to a very conliderable degree of accuracy. As the 
fhip was receding from the fhore, lefs and lefs of its malts 
became vifible ; and, continuing my obfervations, in or¬ 
der to difeover whether any, or what, variations might 
take place, I found that, as the fhip defeended, the images 
B, C, afeended; but, as the fhip did not fink below the 
horizon, I had not an opportunity of obferving at what 
time, and in what order, the images would have vanifhed, 
if the fhip had fo difappeared. 
“ Being defirous of feeing whether the fame effedt was 
produced on the other fhips which were vifible, I directed 
my telefcope to another fhip A, fig. 3, whole hull was juft 
in the horizon xy; when I obferved a complete inverted 
image B, the mainmalt of which juft touched that of the 
fhip itfelf. In this cafe, there was no fecond image as be¬ 
fore. The fhip A moving on the horizon, B continued 
to move with it, without any variation in its appearance. 
“ The next fhip I directed my telefcope to, was fo far 
on the other fide of the horizon xy, as juft to prevent its 
hull from being feen, as is reprefented by A, fig. 4. And 
here I obferved only an inverted image of part of the fhip - 
the image;/ of the topfail, with the mail joining that of 
the fhip, the image x of the top a of the other malt, and 
the image 2 of the end C of the bowfprit, only appearing 
at that time. Thefe images would fuddenly appear and 
di (appear very quickly after each other; find appearing be¬ 
low, and running up very rapidly, fhowing more and lefs 
of the mails at different times, as they broke out; refem- 
biing, in the fwiftnefs of their breaking out, theihooting 
out of a beam of the aurora borealis. As the fhip was de- 
fcending on the other fide of the horizon, I continued my 
obfervations on it, in order to difeover what changes 
might take place ; when I found, that, as it continued to 
delcend, more of the image gradually appeared, till at laft 
the image of the whole fhip was completed, with their 
main mafts touching each other; and, on the lhip de- 
feending lower, the image and the lhip feparated ; but I 
obferved no fecond image, as in the firll cafe ; the fecond 
-image however might probably have appeared, if the fhip 
had continued to def'cend. 
“ O11 moving my telefcope along the horizon, in order 
to examine any other fhips which might be in light, I ob¬ 
ferved, juft at the horizon xy, in fig. 5, the top a of the 
maft of a fhip ; arid here an effedt was obferved which had 
not been before difeovered ; for there was an inverted 
image B, vertical to a, an creel image C, both of them 
very perfedt and well defined, and an image v w of the lea 
between 
z 
