On Tue, 26 Jul 2011 14:30:32 -0700, harry wrote:

The only way I can think of determining water level without a sight
glass, would be by having trycocks above and below the aimed for water
level. Even so, as the water would flash off on release, it would be
hard to tell.
So what have they got on preserved locos? (Early ones I mean)

Locomotion (1825, preserved 1850) has a two trycocks. The Hedley
(Wylam) engines don't have anything at all at the driver's end. I've
not found a photograph that shows the fireman's end well enough to
be sure whether there's anything there or not (Puffing Billy ceased work in
1862, Wylam Dilly had an extension to the fireman's end of the boiler
added in the 1860s and ceased work about 1865 - if anyone in London
or Edinburgh wants to pop into their respective museums, take a
picture of the fireman's end of these engines and upload it, then
it'd be appreciated!).

Side- and end-drawings of the Killingworth Stephenson engines (from
Wood) show no try-cocks at all, not even in the 1831 edition of the
book. The drawings are very detailed, and everything else seems
to be present and correct.

Stephenson's Billy got a new boiler in the 1870s, so although it's
an early engine the boiler practice is of a much later era.
Agenoria (1829, ceased work in the 1860s) has at least one trycock.
I've never seen a picture of the backhead of Invicta (1829),
though that end of the engine was subject to much modification
later in life.

Ahah. A breakthrough! Bailey and Giltheroe's book on the archeology
of Stephenson's Rocket state that it is fitted with a gauge cock
(try-cock) - so one of them - AND sight-gauge holes (all on the back-head).
A contemporary account says:
".. and there is a small glass tube affixed to the boiler, with
water in it, which indicates by its fullness or emptiness when
the creature wants water..."
So Rocket had a gauge-glass when running on the L&M.
Isaac Boulton - who saw Rocket running at Rainhill - says that there
wasn't a gauge glass there at first. Could it have been retro-fitted.
Northumbrian, built in 1830, certainly had a gauge glass from new.
More complication: there are holes (plugged) for a gauge glass on
the RH side of Rocket's boiler. Could it be that the first gauge
glass (the first locomotive gauge glass of all?) was mounted there, and
only later moved to the backhead, after the boiler and firebox were
modified to work with a higher water level (which involved fittng
a dome, amongst other things - Rocket having been stated to have
problems with priming..)
There are twin try-cocks on the front (chimney-end) of the boiler
of Rocket (why at that end?).

OK. Hypothesis. The Wylam engines and the Killingworth engines
operated on short runs - 2-3 miles maximum. Could it be that
for such short runs the practice was to put a certain amount
of water in the boiler at the start (one known from experience
to be 'what she went well with'), then top up again at the end
of the run?
On the S&D (Locomotion) the runs were much longer, so something
was needed to allow the crew to check boiler levels en route
- thus the two try-cocks.
Presumably something better still was thought necessary for the
higher speeds and longer continuous runs that Rocket and her kin
would have to do, so the gauge-glass was introduced (there may
also have been worries about the multi-tube boiler being more
vunerable to water level issues).
I still can't figure out why Rocket has the two try-cocks at the
chimney end and only one at the back, though...

--
From the Model M of Andy Breen, speaking only for himself