Perran Wharf near Truro - WJH intends to try 'some magnetic experiments in some of our mines - in order to ascertain if possible - whether there is any difference in the intensity of terrestrial magnetism - underground and at the surface - and whether the presence of electricity to any great amount can be detected'. He has not seen WW's paper on pendulum experiments at Dolcoath mine - 'allow me therefore to enquire whether the differences seemed to make any approximation to a periodical variation - say diurnal - as the pressure - temperature or moisture of the atmosphere - or whether the rise of the water in the mine which took place during your experiments seemed to affect your apparatus in any degree'. He would appreciate any information WW could supply.
Add. MS a/206/80
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1 Aug. 1829
Part of Additional Manuscripts a