Heath Curve-bar sextant compared with Plath

28 01 2010

The preceding posts covers “A C19 Sextant Restoration” , “Making a Keystone Sextant Case” and “Restoring a C. Plath Drei Kreis Sextant” .

Shortly after I completed the restoration of a Plath Drei Kreis (three circle) sextant I was given the opportunity to restore an English sextant from the same period, a Heath Curve-bar sextant, so called because the frame appears to be made up of thin, curved bars (but is in fact a bronze casting). This one received a certificate from Kew Observatory in 1898 and in addition to being named “Heath and Co., Ltd.,  Crayford, London S.E.” also carried the owner’s name in full engraved in on the limb. As was usual in 19th century instruments, the maker’s name was in copperplate script rather than in the later type.

Like the Drei Kreis, the arc is divided to read to 10 seconds with the vernier and the radius of the arc is much the same at about 165 mm.

Apart from there being widespread verdigris, this sextant posed no particular problems as everything was intact, with no missing parts, so I will not bore you with the details of restoration. It was a simple matter of dismantling it to its parts, cleaning off verdigris, polishing screw heads and re-lacquering. The case needed only filling of a large shrinkage crack in the bottom and cleaning up the handle and latches. Unfortunately, at some time the arc had been polished, so the divisions are in places rather hard to read. While some nineteenth century sextants were chemically blackened, I found traces of black lacquer on the little caps that cover the adjusting screws. The next photo shows the instrument cleaned and reassembled while I waited for the owner’s instructions about re-lacquering. 

This photo shows the finished article:


 I thought this an ideal opportunity to compare an English with a German vernier sextant of approximately the same period, at the turn of the twentieth century.

In the preceding post I mentioned the complication of the tangent screw mechanism in the German sextant. In the Heath instrument, it is perfectly conventional and typical of a mechanism that seems to have served makers and navigators well for over a hundred years.

In the photo above, the sliding block moves in a curved slide formed by a cut-out in the index arm expansion. The floor of the slide is formed, as in the German sextant,  by a plate that is screwed on to the front of the index arm. The end of the clamp screw bears on a small plate to which is attached the clamp leaf spring. The tangent screw is held captive on the index arm by a bearing. The tangent screw has the familiar squared end, and the knurled knob is attached over the square by a central screw which allows the bearing to be adjusted to remove end play. The squared end prevents rotational forces from being applied to loosen the screw. The sliding block is held against the floor of the slide by a leaf spring, seen in the next photo, of the front of the index arm expansion. The screw that passes through the leaf spring holds the nut on to the sliding block. This spring has the same function as the leaf spring on the rear of the sliding block of the German instrument.

The nut is split. This allows the nut to spring inwards and to grip the tangent screw thread flexibly and remove lost motion between the screw and the nut. When the clamp is tightened, the sliding block is held immobile on the limb and when the tangent screw is turned, the index arm is caused to move via the screw bearing.

While the German arrangement is free from backlash when in good adjustment, if there is any stickiness in the slide, it has the annoying feature of moving in one direction, but not returning by means of the spring pressure. In the other arrangement, if the fit between the tangent screw and the nut is not good, due, say, to wear in the screw, or if the bearing is out of adjustment, then there is backlash in both directions. In both types, if the slide is sticky, movement may be uncertain and jerky. In my view, each defect is as bad as the other, but the English mechanism has the merit of simplicity.

In every edition of his work “Wrinkles in Practical Navigation” (and it ran to 21 editions ) Captain Squire Thornton Lecky wrote “An indispensable condition in a sextant is regidity; flexure is fatal…”, so I thought to make a quick comparison of the two sextants, using the crude set up shown in the next photo. The middle 100 mm of the limb is held clamped firmly in a vice and a dial  indicator, reading in 100ths of a millimetre bears against the end of the index arm bearing. A force of one kilogramme is applied to the index mirror by means of an old  kitchen scale and the amount of deflection read off the dial indicator.

Though the German sextant at 1.5 kg weighed about 250 G  more than the English one, there was little difference in the amount of deflection, about 0.6 mm. Interestingly, two mid-nineteenth century English quadrants of radii 20 mm greater and of about the same weight, deflected significantly less at 0.5 mm for a Spencer, Browning and Co., and 0.4 mm for a Crichton. Late twentieth century die-cast aluminium alloy instruments typically deflected about 0.12 to 0.15 mm in this set-up and bronze-framed ones somewhat more. Allowance must of course be made for the smaller radii of the modern instruments, but even so, the superior rigidity of alloy instruments is clear. Neverthless, right to the end some buyers expressed a preference for the heavier, more expensive but less rigid bronze instruments.




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