As part of a large cataloguing project to research all the NMM's marine chronometers, I am currenmtly undertaking a close study of Harrison's third marine timekeeper, H3 (made during the 1740s). It is proving hugely interesting and revealing as can be seen in the videos which have been tracking my progress.
H3.jpgMarine timekeeper, H3 (ZAA0036)
The aspect I'm studying at the moment is the extraordinary (at least to me) glazed brass case of the instrument, and I'm wondering about 18th century 'instrumental' practice where glazed cases are concerned?
The case (which the timekeeper fits in very closely indeed and which stands just over 60cms high) has a 'top', a middle band (attached to the timekeeper) and a 'bottom', the three parts held together round the middle with 32 screws. The whole thing is incredibly beautifully made, using cabinet-making techniques, and consists of precisely 501 parts, all fitted together mechanically, with no solder anywhere.
The brass panels are just 2mm thick and the four main vertical edges are dovetailed (yes!) all the way from top to bottom with a total of 174 tiny dovetails rather in the way that coppersmiths tie plates together before soldering, but much finer and without solder. The dovetails are so well cut the vast majority cannot be seen, but I show a patch where corrosion and stress has revealed some of them (I have temporarily marked the lacquered surface with felt-tip pen to identify them).
H3 rivets.jpgThe brass panels with dovetails
The structure inside forming the frames for the glazed panels are all pinned and riveted with 425 rivets, and the glass is then puttied in.
The decorative moulded cornice is also 'invisibly' attached all round with pins, disclosed at one corner where the case was damaged in the past and was apparently heated to repair it, not very successfully.
H3 cornice.jpgThe decorative moulded cornice
I wonder whether such large cases are unusual at this period, or are other instruments made in the mid-18th century that are housed in such cases? If so, how are they constructed? I am familiar with the 18th century grand orreries (e.g. those by Wright etc) in the lovely 'cold-frame' type wooden glazed covers, but can't think of anything in metal at this period. If anyone knows of any examples please do get in touch below.