These instruments include all of that smaller class employed by travellers and navigators in determining latitudes and longitudes, and in making surveys.

At sea, the only instrument which can be employed, is some form of the double-reflection instrument, as the sextant, or rather the reflecting circle; and as this is able to compass all the requirements which may be made of it there, great advantage will result in economy, portability, and despatch, if it can be made also to serve the purposes of a traveller by land.