i802. 
Tt has been long doubted, whether 
Archimedes did really burn the Roman 
fhips of Marcellus at the fiege of Syra- 
cule with a large mirror, as no fuch feat 
could ever be performed by fucceeding 
opticians. Ifhe did, no doubt it mui 
have been effected by a vaft number of 
{mall mirrors placed on a large concave 
furface,or frame of wood, foas to have their 
reflected light thrown together at a greater 
or lefsdiftance by means of adjufting fcrews. 
This was the manner in which Baffon 
concentrated a vat quantity of light 
thrown from a concave cafe or fruftum of 
a globe filled with 400 mirrors; but, as 
the focus mult have been the fize of one of 
his mirrors, the condenfation of light was 
not fufficient to burn intenfely, without a 
convex lenfe to concentrate the light fo 
affembled more toa point. In this focus 
various metals were melted, diamonds 
difperfed, &¢c. and it ‘has only  re- 
mained a curiofity in the Botanic-garden 
in Paris fince his time. 
Other artifts have lined the fruftum of 
a globe with ftraw, and other polifhed re- 
fle&ting fublftances ; but we do not find 
thefe contrivances either ufed for burning 
of fhips, or lighting them on their way: 
they were philofophical curiofities, and, 
like other novelties, amufed the curious 
foratime. But this is one among thou- 
fands of inftances, how a theorem in ma- 
thematics, or an invention in mechanics, 
may at firft feem a mere toy for the 
mind, and applicable to no earthly pur- 
pole, yet, in time, may be found to con- 
tribute to the eltablifhment of truth, or 
the general happinefs of fociety. Did not 
a falling apple firft fuggeft the law of 
gravity? Did not a fhepherd difcover 
the load-ftone? And the diffection of a 
frog the galvanic influence? We fhould 
not laugh at experiments that fail, or at 
thofe which feem to lead to nothing—No 
trial is loft! 
But to return. The firft ufe of con- 
cave-mirgors, I remember to have feen 
(about thirty years ago), to direct fhips, 
were at Liverpool and Newcaftle: into 
the firft, the road is very intricate, through 
fands and fhoals, from Hoy Lake to the 
Black Rock. ‘To direét the mariner in 
the night, that ingenious and ufeful dock-~ 
mafter, Captain Hutchinfon, placed po- 
lihhed copper mirrors in light-houfes, 
facing each turning of the Channel, fo 
that the pilot made way towards the firft 
light he faw, till he faw another to his 
fight or Jeft; he then turned immediately 
3 
Uje of Mirrors 
as Ship-lights. ate 3 
towards the fecond light, and purfued that © 
channel till another light broke in upon 
him, to which he then directed the fhip. 
Thus is the harbour obtained by night as 
well as by day. At Newcaltle, the Tyne 
flows perpendicularly into the German 
Ocean, and veffels in the night might fail 
paft its mouth, were it not for a copper- 
mirror that throws out a light into the 
fea, in the direction of the river, fo that as 
foon as that light is beginning to be 
crofied, the fhip turns towards it, and is 
directed by it into the river. 
Thefé were in ufe in the year 1770, 
and are, no doubt, at this time, greatly 
improved. But this thews that mirrors 
were employed for navigation long before 
1779 and 1787, the date of Mr. Ezekiel 
Walker’s invention, as fet forth in your 
impartial Magazine. Befides, the French 
have ufed mirrors at the mouths of their 
harbours time out of mind! Far fromdi- 
preciating the merit of Mr. E. Walker’s 
mirrors, I greatly admire them; and he 
deferves much praife for having aflilted in 
bringing the huge mirrors of Buffon and 
Villette into general ufe, both without 
and within doors; and certainly he who 
can increafe light, even as much as witha 
farthing-candle, deferves well of a come 
mercial country. 
J am led into this refleCtion, by having 
lately failed by the Scilly Light-houfe, 
where I was much ftruck with an inter- 
mitting light, that at once tells you what 
light it is, by difappearing every fecond 
minute. This 1s alfe 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 {preads nor 
converges, but darts a cylinder of lighe 
to avait diftance. This light confifts of 
fix round mirrors placed round a feventh, 
each twenty-two inches in diameter; eve- 
ry 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 horizon, on 
a fhaft united with a machine below that 
turns the whole round every two minutes « 
hence a cylinder of light, five feer and a 
half diameter, (weeps the whole horizon, 
and, by its intermiffion,; cannot be mif. 
taken for any other. This powerful light 
I find has been copied in Spain, on the 
Skerries, on the Ifle of Wight, &c. fome 
ftationary, and fome with motion, without 
confulting the original inventor. Upon 
feeing Mr. Ezekiel Walker’s account of 
his mirrors, I concluded he mutt have 
brea 
