NavList:

Forty-three years ago I paid a visit to the National Maritime Museum in Greenwich. At that time, there was an exhibition about the voyages of Captain James Cook. I looked in wonder at the fine divisions on the arc of a sextant made by Jesse Ramsden and pondered how they might have been made. This led me to learn about engineering workshop technology and I gradually acquired some practical knowledge of dividing techniques. In the winter of 1990, in the Science Museum in London, I was able to examine a dividing engine of the type invented by Jesse Ramsden, one of the foremost instrument makers of the eighteenth century and a few days later stumbled across his prototype dividing engine in an ill-lit corner of the Musee des Arts et Metiers in Paris. After intermittently collecting and studying papers, a process much accelerated by increasing familiarity with the internet, I think I may perhaps now have something worthwhile to say about Ramsden’s circular dividing engine (he also made a linear dividing engine). I hope it will be of interest to navigators, as without dividing engines, the sextant upon which the navigator until recently relied would not have reached its final accuracy. It may also be of interest to engineers and students of the history of technology.

It is ill-adapted to paper publication so I offer it to readers and members of NavList. There are two linked documents. The first is Ramden’s text and plates with lettered references for each page. The other is a three part document. Part I is a commentary linked page by page to the references in the text document. Part II comments on Ramden’s screw-cutting lathe, an essential precursor to his dividing engine, and Part III gives a contemporary account of his life. If you open both documents you can flit back and forth between the two. I find it easier to view the two on a screen that is split into two side-by-side panes at 75%, increasing to 200% when wishing to view details of the beautiful engravings that accompanied Ramsden’s text. To view split screens in Windows, open both documents. While viewing one, press and hold <CTRL> and right click on the tab of the other at the bottom of the screen. A menu will pop up. Select <Tile vertically>.

I have formatted both documents so that they can be printed like “proper” papers, though my lack of expertise in this area and a desire to make the figures as large as possible accounts for occasional gaps in the text.

Bill Morris
Pukenui
New Zealand.

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