V. — Description of the Anglometer, an instrument for working Lunar Calculations. By Captain C. Cowles.
The instrument represented in the accompanying drawing (Plate XIV.) was invented by me some years ago, with a view of reducing to easy practice one of the many methods laid down for solving the " Lunar Problem." The method I allude to will be found in Dr. Kelly's " Practical Introduction to Spherics and Nautical Astronomy." In speaking of it, he says in his preface to that work, (page 10.)
" The manner of finding the longitude by the lunar observations is explained at some length in an easy familiar way, and the principles are illustrated by stereo- graphic projections, whence rules are deduced for estimating the correction before hand. As this subject has not been attempted before, it required the more consi- deration, which has led to the discovery of a method of solving the problem by pro- jection of four right lines from the plane scale ; and although this method cannot be insisted on as perfectly correct, yet considering the complicated nature of the problem, and the great simplicity of the projection, the degree of accuracy must be matter of surprize rather than animadversion, as it will be found sufficiently correct for the general purposes of navigation. Where perfect accuracy is required, this method will be useful as a guide or check to calculation : and it is hoped, that the extreme facility of the operation may tend to render the practice of taking lunar distances more frequent among the generality of seamen."
Simple as the above method is, there are difficulties, which oppose themselves to its general adoption ; for instance, to perform it with any degree of accuracy, it is necessary to have a tolerably smooth surface to work on, good compasses and scale, with a hard well-pointed pen- cil ; on board ship one or other, and sometimes all these requisites are wanting, and when present, it must be recollected, that even amongst intelligent and well educated seamen, there are many who are not aufait at the pencil and scale. The object of this instrument is to remedy these inconveniences as far as may be, as it performs the work of paper, compass, scale, and pencil : it has been used by many persons, among whom I may mention Captain Laws, of H. M. S. Satellite, from whose concurrent reports on its merits, as well as from my own experience, I should say, that " its accuracy is matter of surprize ;" that under all cir- cumstances, it is a very desirable check on calculation ; and, when the " distance" is not great, its accuracy may be depended on without calculation ; and lastly, the certainty of being able to determine the longitude, in the course of a few minutes, is a strong inducement to take lunar distances when otherwise they would not be attempted.
Plate XIV. exhibits a perspective view of the instrument as made up by myself with the rude means at my command; the method of using it to clear the lunar distance is as follows :
