Letter | Published:

Sextants

Abstract

IN your review of the “Encyclopædia Britannia” published last week I notice that reference is made to an article on navigation by Capt. Moriarty, and attention is called to the very serious error in sextants arising from false centering. Having had some experience in the examination of these instruments, I can practically testify to this most important defect. Only a week or so since two sextants were received here for trial, one of which belonged to a captain of the mercantile marine. In both instances, although the mirrors and shades were good, yet the arc error due to false centering was excessively large, increasing from 0 at 0° to + 7′ at 60°, while at 90° it amounted to 10′. Surely this must be a serious matter to navigators, but, as you point out, for the small fee of five shillings persons ordering a sextant may direct the maker to send it to the Observatory, where suitable apparatus is arranged not only for examining the arc but also the mirrors and shades. It is only fair, however, to say that when instruments are sent direct from the makers we do not often have occasion to reject one. Indeed, superior sextants by first-class makers rarely have an error exceeding 1′ of arc, and often not more than 30″, but how few these are in comparison with the hundreds of inferior instruments that pass into the hands of the public without being tested.

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