On 6/11/2010 9:36 AM, Adrian Stott wrote:
> Trevor Sheppard
> <[email protected]> wrote:
>
>
>> Having just read an article about a home burning to the ground because
>> of the sun's rays being focussed through a glass paperweight (one of
>> those glass balls - which many of us probably have on our windowsills at
>> home..) I was surprised at how close to<my> home this actually came.
>>
>> I have one of those glass roof lights over the engine hole - I think
>> they are called something similar to 'bullseyes'? - and when I went to
>> open one of the small drawers in there today it was stuck tight.
>> Thinking it was the plastic carrying handle on the top (useless anyway
>> as it is screwed to the bulkhead) I gave it several 'tugs' and,
>> eventually, it came free - though no longer in a useable state .. lol!
>>
>> Closer inspection showed that the drawer had become welded to the rack
>> (both being plastic) in one corner. No chance it was down to anything
>> other than the, evening, sun - but it made me very aware of how much
>> these lights can focus sunlight, and how hot the focal point can get :-(
>>
>> I'd guess that most people will not experience this effect, unless -
>> like me - they have something quite close to the roof light .. but its
>> certainly worth checking out.
>>
> I've heard tell that this problem can occur only if the bullseye is
> incorrectly installed, which is a common mistake.
>
> AIUI, the bullseye should be installed with the curved (convex) side
> *down*. This lights the interior better, by dispersing the light. If
> the bullseye is installed the other way up, it concentrates the light
> (and heat) on a point in the interior.
>
> Again AIUI, people tend to install them upside-down because that way
> water can't accumulate on top of them and eventually leak into the
> interior.
>
>
Now this is all very interesting, but our original bullseye faces up,
and does in fact heat up the clutch rod, and if some silly so and so
puts a towel on that rather that the towel rail it attempt to catch
fire, on a sunny day. Now we have always assumed that it should have
pointed up, because that's how we found it, or more to the point the
piece of lead which mounted it, as Taff had a new roof fitted in the
late 1960's but now I've looked at the old pictures I can't actually
swear its correct. How very interesting. More research required. Also it
was worn on the top of the bull, which again implied it was on the outside.
--
cheers Ian Mac
NB Spey
