John, Actually, my father does a similar dial to the one in the photograph which has longitude adjustment. Having said that, the method he uses is to have a longish, narrow slot cut in the time scale through which pass 2 threaded short rods. Gnurled nuts screw onto these and hold the scale firmly in position. I dare say that the adjustment is not as fine as could be achieved by a rack device, however, once the dial is set up this factor should not need to be changed. Whether the investment in time is worth it, given the stated difficulties in producing a curved rack, I'm not sure. I should have thought that the sliding scale could be set to the nearest minute (time) which would be the equivalent of a quarter of a degree of longitude. Of course the easiest way to set it would be to pick a day when the EoT was zero, and with the dial correctly orientated north, slide the longitude adjustment as appropriate so that the shadow was on the 12 o'clock line at noon. If this were done as carefully as possible then the resulting settup would be as acurate as the dial could be read!
As far as the curved rack goes, would it be possible to buy one whose teeth are angled (//////) and bend this along its length to put it on the outside of the U. I should imagine that you could then choose a worm of a suitable size and position it at a suitable distance such that only the tips of the teeth of the rack engage in the worm. The spaces in the rack would then be unimportant. My knowledge of such things is restricted to playing with Meccano as a child, so I don't know how easy it is to cut angled teeth for example, however my Meccano set had cogs with angled teeth so I suppose it can't be too difficult or expensive! Let me know what you think. David Higgon London
