The Theory and Practice of Finding the Longitude at Sea or Land:

First edition of this important and scarce work on Longitude. The Scottish teacher of navigation, Andrew Mackay (1758-1809), 'corrected the latitude of Aberdeen, estimated the longitude by comparing timings of Jovian satellite phenomena with Greenwich, and rated chronometers. Meantime he earned his living teaching mathematics, geography, navigation with lunars, surveying, and fortification' (ODNB) and the current work gained him the 'thanks of the boards of longitude of England and France' (ibid.). In his Preface, Mackay makes an interesting reference to the horologists John Harrison, Larcum Kendal and John Arnold 'If a chronometer or timekeeper could be constructed, so as to go uniformly when placed in different position, and under different degrees of heat, then would this method of finding the longitude at sea be a most valuable acquisition to the navigator... every person acquainted with the principles of watchmaking must highly admire the accuracy of Harrison's timekeeper. Those made by and Kendal are also excellent; but it is to be hoped, that instruments of this kind may be still farther improved, and may be afforded much cheaper than at present, for the high price is a very great objection to them, and very much prevents their being generally used'.

The list of subscribers runs to approximately 530 names (taking in the region of 610 copies) and although largely Scottish, includes Sir Joseph Banks, and the astronomers Nevil Maskelyne and William Herschel. The remainder are predominantly from Mackay's home town of Aberdeen, listing many shipmasters, mathematical instrument makers, and merchants. Copac locates copies at 12 British and Irish institutional libraries; OCLC adds a little over a dozen worldwide but this is very scarce in commerce, and only a handful of copies have appeared at public auction since 1966.