A tedious problem which confronts many a yachtsman is determining the deviation of his magnetic compass. He can hardly justify the expensive services of a professional compass adjuster, so this necessary but difficult operation must be carried out by himself. Should he decide to use bearings of shore objects when swinging ship he is faced with the difficulty of observing these bearings on his compass. A small-boat compass is hardly likely to come equipped with azimuth mirror and sight vanes. The alternative equipment for taking the bearings is a pelorus, but the expense of this instrument could hardly be justified for such rare and intermittent use.

The favourite way out of the difficulty appears to be to use a hand bearing compass. This method suffers from two disadvantages in particular:

(i) The hand bearing compass should be situated in precisely the same position as the main compass if it is to reproduce the latter's readings precisely. Not only is this impossible to achieve in practice – the nearest they can be brought together is side by side – but when brought close together their mutual magnetic reaction can produce errors of an unknown amount.

(ii) The use of bearings to shore objects naturally entails being offshore. Stationary small boats are notoriously unstable observation platforms and even a light chop would make it impossible to read the main compass with the accuracy necessary for a good deviation card.