O14 
from the top of Severndroog Caftle, on 
Shooter’s Hill. 
“ The. Corporation therefore requeft the 
favour of gentlemen, who refide within a 
cirele of thirty miles of that objet, to take 
notice of the appearance and magnitudes of 
the different lights on the nights when the 
refpective experiments are made, and to 
favour the Secretary with any obfervations 
that may occur on their comparative merits, 
«* Due notice will be given of the nights 
on which the caftle will be lighted.” 
What fuccefs has attended thefe lauda- 
ble endeavours of the above-mentioned 
corporation to improve light-houfes, I 
have not been informed: but if the light- 
houfe at Scilly has been ereéted fince thofe 
experiments were made, fome judgment 
may be formed; for that light is under 
the direétion of the Trinity Houfe. 
The Scilly light-houle, Nauricus in- 
forms us, “**is alfo a light of mirrors, but 
they are of copper, plated with filver, 
and polifhed in the curve of the parabola, 
by which their light neither fpreads nor 
converges, but darts a cylinder of light 
toa vaft diftance.” This light confifts of 
fix round mirrors, placed round a feventh, 
each twenty-two inches in diameter ; 
every mirror having an Argand’s lamp in 
its focus, fupplied with oil from behind. 
The frame in which thefe mirrors are 
fixed ftands perpendicularly to the hori- 
zon, on a fhaft united with a machine be- 
low, that turns the whole round every two 
minutes. Hence, a cylinder of light 
five feet and a half diameter fweeps the 
whole horizon.” 
Ganganelli fays, in one of his letters, 
‘¢ that it is eafy to perceive in reading a 
book, even a moral one, whether the au- 
thor be a mathematician or not. And that 
he was feldom deceived in this obfervation.”’ 
Without poffleffing the penetration of 
Ganganelli, I think I may venture to fay, 
that had Nauticus been pofleffed of a mo- 
derate fhare of mathematical knowledge, 
he would not have given us fucha deferip- 
tion as this. : 
For thofe mirrors to refleét parallel rays 
of light, each muft have a lamp no larger 
than a mathematical point. But, let us 
fuppofe fora moment, that thofe mirrors 
_ have this property of darting a cylinder of 
light to the horizon, which may be about 
fifteen miles difiance; then, as the frame 
in which the mirrors are fixed turns round 
‘once in two minutes, this Will-with a- 
wifp at the horizon muft travel over more 
than ninety-four miles in that time, or at 
the rate of four miles in five feconds; and 
as this light is only five feet and a half in 
On Reflecting Lightehoufes. 
[April 4, 
‘diameter, even at the horizon, it would 
not continue in fight fo an eye placed 
there, much longer than =45 part of a 
fecond. But we are informed, that each 
mirror in that houfe has an Argand’s 
lamp fixed in its focus, confequently, 
from the well-known principles of optics, 
the rays of light reflected from it mult be 
in adiverging flate; and a reflegted light, 
to be ufeful at fea, muft diverge to a 
certain degree, that the whole furtace of 
the water, between the light-houle and 
the horizon, may be illuminated. 
. The Scilly light-houfe may, for aught 
I know to the contrary, be perfeét in this 
refpeét ; but upon the whole it is far from 
being fo good as Nauticus fuppofes, or fo 
good. as might have been expected from 
the genius of Mr. Adam Walker, affifted 
by the purfeof the Corporation of Tri- 
nity Houfe. The ufe of filver for a re- 
fiecting furface is certainly an imperfec- 
tion of no fmall consequence. Silver is 
fo much inferior to looking glafs for this 
purpefe, that, for philcfophers to prefer 
the former to the latter, appears to me 
very unaccountable. It is my humble 
opinion, that there is nota lady in the 
United Kingdoms who would prefer a 
filver difh to a looking-glafs, when fhe 
‘is-inclined to fee a pertect image .of her 
face. 
It may be of ufe to thofe who may 
hereafter copy the light-houfe at Scilly 
to know, that the figure in which thofe 
mirrors are fixed is as good as any other, 
but not better. Had they been fixed in a 
{quare, a parallelogram, or “a triangle, 
their effect would have been juft the fame, 
for they would have appeared as one fingle 
light at a very fhort diftance; nay, had 
they been placed ina right line, their ef- 
fect would have been ftill the fame at three 
miles diftance. For, it is known from 
experiment, that two reflectors, made of 
tin, placed more than ten feet afunder, ap- 
peared at. three miles diftance as one fin- 
gle light, and required to be feparated 
from each other more than nineteen feet 
to appear two diftiné lights, when viewed 
at the fame diftance as before. 
When it is confidered how great the 
dangers gre to which fhips are expofed 
near the fhore, particularly in the dark 
tempeftuous feafon of the year, and that 
good licht-houfes conduce greatly to the 
fafety of property, and the prefervation of 
the lives of feamen, I hope, that nei- 
ther Mr, Adam Walker, nor NauTICUs 
will conceive any remarks that I have 
made upon the light-houfes at Scilly, as 
in the leaft degree perfonal. The cha- 
~ vatter 
