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I was in London last month and made arrangements (with the assistance of 
Geoffrey Kolbe, thanks again) to see the examples of Bygraves and the HR 
series held by the Science Museum and by The National Maritime Museum. I 
attach two photos showing my hot little hand holding examples of each.

The Bygrave I saw was a Mk II serial number 105, the same one shown in Van 
Riet's paper. I measured the diameters of the scales, actually their 
circumfrances using a tape measure, and my numbers are very slightly 
different than his. I believe my measurements are accurate to one-half of a 
millimeter.

The circumfrances were:

Cotan   174mm
Cos     187mm
Cursor tube 203mm
Knob at bottom of cosine scale  207mm

Dividing by Pi gives diameters of:

Cotan 55.4mm
Cos   59.5mm
cursor 64.6mm
Knob 65.9mm

I am attaching two photos showing two details of the construction. One is a 
photo of the cotan scale showing that it was printed on paper and then 
wrapped around the tube. You can see the gap that has developed over time. 
The second shows a detail of the long pointer, it is bent downward to be near 
the cotan scale to eliminate parallax.

The National maritime Museum has three examples of the HR1 and one example of 
the larger HR2. The three HR1s were in cans and the HR2 had a cover and was 
obviously meant to be mounted on a desk. The first two HR1s I examined were 
in perfect shape but the third one (the one I am holding in the picture) was 
in poor shape. I am attaching two photos of the HR2 one showing the scale 
revealing the same construction method used in  the Bygrave, a paper scale 
glued around a tube.

It was a thrill for me to actually see these instruments. My first impression 
was that they are very big compared to the one I constructed which is only 
one and a half inches in diameter.

gl


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