Cary-Porter Balloon octant
Cary Porter Balloon Octant
Obtained from the UK
Often called a balloon sextant, this instrument can actually only measure to 90 degrees and in fact is not even a sextant but technically a clinometer. It was used for astronomical observations though and can be seen as the predecessor of the bubble sextant. The telescope has a mirror (like the hand held levels) that looks towards a small bubble level mounted on the index limb. This way the vertical angle relative to the horizontal can be determined. Like a drum sextant this instrument has a rim with teeth as well as a micrometer and quick release clamp.
On top of the telescope there are a set of filters. The instrument can be lighted for use at night and takes what seems to be a 4.5V block battery in the handle. Besides an push button the handle also has a rheostat for adjusting the amount of light. The latter is something I have not seen on other sextants before. The instrument comes in a box with a trade label for Cary Porter Limited in the lid with a Regent exchange telephone number. This would date it somewhere between 1900 and the end of the company in th1920's.
The instrument has an unusually low number, 4, stamped on it. If this would have been the first of a long series remains to be seen; there is very little information on these instruments available.
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Cary Porter Balloon Octant
Obtained from the UK