1803. ] 
temperature was only 3°, and the evapo- 
ation, to counteract this excefs of warmth, 
produced 3° of cold, the refraction vifible 
was 5’. Hence it fhould feem that under 
fome circumftances, the folution of water 
in the atmofphere cautes a decreafe in its 
refractive power. . . 
The obje&t made ule of in thefe expe- 
riments,as fhewing belt the quantity of re- 
- fraétion, was an oar dipped in the water at 
the greateft difcernable diftance, or fome 
other line equally inclined ; and the angle 
meafured was taken from the point where 
the inverted image js terminated by the 
water, to that oart of the oar itfelf which 
appears directly above it. 
As the reiult of all his obfervations, Dr. 
Wollafton concludes that the quantity of | 
refraction cver the furtace of water may 
be confiderable where the jand is near 
enough to influence the temperature of the 
air. But at fea, fo great differences of 
temperature cannot be expected ; and the 
increafe of dip caufed by. this variation. of 
horizontal refraétion is not fo great as in 
the confined courfe of a river: it may, 
however, be fubje& to an equal diminu- 
tion from an oppofite caufe, and the hori- 
zon may even become apparently elevated, 
and therefore the error in naptical obfer- 
vations, arifing from a fuppofition that it 
is invariably according to the height of 
the obferver, ftands in need of correétion. 
From Dr. Herfchel’s Osfervations of ihe 
tranfit of Mercury over the Sun, Nov. 9, 
180z ; we learn that the appearance of 
the planet, during the whole time of its 
emerging from the Sun, remained well de- 
fined even to the laft; that the following 
limb of Mercury remained fharp, till it 
reached the very edge of the fun’s dik; 
and vanifhed without occafioning the 
{mallet diftortion of the fun’s limb, in 
going off, or fuffering the leaft alteration 
in its own figure. 
During the tranfit, Dr. Herfchel exa- 
mined the ‘appearance of Mercury with a . 
view to afcertain its figure, but he could 
not perceive the leaft -deviation from a 
{pherical form, fo that, unleis its polar 
axis fhouid have happened to be fituated, 
at the time of obfervation, in a line 
drawn from the eye to the Sun, the pla- 
pet cannot be materially flattened at its 
oles. 
In obfervations aud experiments relat. 
ing to the caufes which cften affec? mirrors, 
Jo as to prevent their fhowing objects dif- 
‘tinéily; Dr. Herfchel jays, “ It is. well 
known to altronomers, that telefcopes will 
act very differently at d.fferent times, 
The caufe of the many difappointments 
they have met with in their obiervations, 
Proceedings of Learned Societies. 253 
is, however, not fo well underftood.** 
His own long experience enables him, he 
thinks, to aflign the principal caufe ot the 
difappointments to which aftronomers are: 
fo often expofed, and he lays it down as, 
an axiom: ‘* That in order te fee well 
with telefcopes, it is required that the 
temperature of the atmofphere and mirror 
fhould be uniform, and the air fraught 
with moifture.”” 
Hence a frofe after mild weather, or a 
thaw after a froft will derange the perfor- 
mance of mirrors, till the temperature of 
the mirror accommodate itfelf to that of 
the air. For without fuch an uniformity 
with the open air, in the temperature of the 
mirror, the tube, the.eye-glaffes, and even 
the obferver, be obtained, we cannot expect 
to fee well. 
When the froft becomes fettled, the 
mirror foon accommodates itlelf to the 
temperature. This explains the reafon 
why no telefcope jufi brought out of a 
warm room can act properly. Nor can 
delicate obfervations be made when look. 
ing through a door or any confined place. | 
Windy weather, which occafions a mix. 
ture of airs of different temperatures,can- 
not be favourable to diftin& vifon, The 
fame remark will apply to Aurore Be, 
reales, when they induce a confiderable 
change in the temperature of the atmo- 
fphere.. The warm exhalations from the. 
roof of a houfe, in a cold night, muft dif- - 
turb the uniformity of the temperature of 
a {mall portion of air; fo that Stars which 
are over the houfe, and at no confiderable 
diftance, may be affected by it. 
Sometimes the weather appears to be 
fine, and yet the telefcopes will not a& 
well. This may be owing to drynefs oc- 
cafioned by an eatterly wind; or to a 
change of temperature arifing from an 
agitation of the upper regions of the at- 
mofphere: or from both thefe caufes 
combined. H 
If moifture in the atmofphere be necef- 
fary, dampnefs, hazinels, and even fogs, 
to a certai# degree, are favourable to_ce- 
leftial obfrvations. 
Thelg inferences are drawn from a vaft 
number of ob{ervations made with fpecula 
of undoubted goodnefs, principally dur. 
ing the years 1779, 1780, 1781, 1782, 
2733. 
a 
AMERICAN PHILOSOPHICAL 
SOCIETY. 
\\ BAUDRY DES LOZIERES has 
e given to this Society a curious 
and highly intereftinz Memoir on Ani. . 
mal! cotton, or the “ Inlect Fly-carrier.”’ 
Every inhabitant of the Welt Indies, fays 
a this 
