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George Huxtable wrote:

> And again, I'm told that Halley wrote, in Phil. Trans. 37, (1731), 185-95,
> that he obtained his longitudes at sea by observations of "a near Transite
> of the Moon by a known fix'd star", using "a five or six foot telescope,
> capable of showing the appulses or occultations of the Fix'd stars by the
> Moon, on shipboard, in moderate weather." How he managed such a telescope, I
> have no idea, but at a guess it must have been suspended from the rigging at
> the far end.

Anyone with experience at sea knows that you can't steady an instrument
by coupling it to the vessel - you have to isolate it.  That's why
Galileo suggested nested hemispheres as an observing platform for his
celatone and Maskelyne tested a gimballed chair.  Suspending a telescope
from a yardarm would be futile.

At the end of the 17th century, telescopes were typically made with
pasteboard covered with shagreen, vellum or paper.  Lens cells and
fittings were brass but the overall weight of such a telescope was low
compared to a brass tube.  In small apertures, such a scope would be
very awkward due to its size, but not due to its weight.  Halley could
have managed one for lunar occultation or appulse observations.

On the other hand, Newton's instrument called for brass and a stiff tube
would be required to support the brass plate.

====
NOTE: I goofed on the size calculation in a previous post.  The diagonal
grid (as calculated by Nicolas) was 1.5mm x 1.27mm at 1/12 arcmin.
Dropping to 1/6 and reducing the concentric rings would result in 1.5mm
x 2.54mm.  Dropping that to 1mm max would end up with 1mm x 1.7mm with a
divisor of 1.5.  Applying that to the plate size - 3.75 ft. max or 45"
yields a octant plate with a max size of 45/1.5 = 30"!!!  That's around
350 sq. in. of brass with its centre of mass a little past halfway to
the objective lens end of the 4 foot brass tube.
====

Using Newton's instrument, as described in his notes, would be basically
impossible.  It's just too big and heavy.  Unless some kind of smaller
instrument that yields a suitable accuracy can be identified, it remains
that the instrument that Halley used was unknown.

Charles H. Cotter,  "The Mariner's Sextant and the Royal Society" Notes
and Records of the Royal Society of London, Vol. 33, No. 1 (Aug., 1978),
p. 31:
"There is no evidence, however, that Newton's instrument materialized,
let alone that it was tested at sea."

Mike


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