An x-ray of a wheel, in this case the great wheel, from John Harrison’s precision pendulum-clock No. 2, reveals some interesting details about its construction, and some hints as to Harrison’s roots as a joiner in a largely rural area. First, the movement is made almost entirely of oak. There are some small components made of lignum vitae, a very dense and waxy tropical hardwood from the West Indies and Central Amercia. Its waxiness means that it has a very slippery, smooth surface. Harrison used them for friction reduction without lubrication and he probably learned about this material from ship’s carpenters in Hull and on the Humber estuary.
The construction of the wheels of the Harrison clock movement is very similar to the construction of the wooden wheels seen in the machinery of wind and water mills.
The grain of the wooden teeth is aligned radially, like sunbeams. For Harrison’s clock wheels and cogs this was done by gluing blanks of wood into a groove cut into the edge of the wheel, and then forming the teeth. If the wheel had been formed from one piece of wood, the teeth at the short grain parts of the wheels would have snapped off. Whilst there is no direct evidence that the Harrisons worked on windmills, the fact is Lincolnshire had hundreds of them, a good supply of wind readily available from the North Sea, to power the windmills that ground the wheat that grew in the flat lands of Lincolnshire. An interesting feature is that Harrison used very slow growing quartered oak for the main body of the wheels. The annual rings are very close together, so the wheel is both very stable and light. The wood for the teeth is, however, from fast growing oak. It is denser and therefore stronger, strength where it is needed. Harrison really knew his onions when it came to materials, an engineering approach to woodworking. My own tests comparing the weights of slow grown and fast grown oak show that the slow grown is a considerable 27% lighter than the fast grown oak. The picture below (picture credit Jeff Darken) is a cog from Harrison’s precision pendulum-clock No. 2. The inner construction is shown in the x-ray at the top of this blog entry. The roller pinions are of lignum vitae. There is an earlier blog entry about this clock, search under Conservation.
A BBC documentary on John Harrison is available on iPlayer until 24/05/2010
And a Yorkshire Post article
The Science Museum’s website included a section on Making the Modern World; the subject of navigation and the technologies developed to enable precise navigation and mapmaking features on the link below:
The Science Museum also has one of Harrison’s earliest clocks, before he began his longitude experiments: