30 Rev. Mr. Emmett on [Jan. 



visible using equal powers. Besides, in these obsei*vations, the 

 simple refractor possesses one great advantage : when the 

 moon, a bright star, or planet, is in or very near the field of view, 

 the field is Jess illuminated than that of an achromatic ; this 

 arises partly from the length of tube required, and partly from 

 the imperfections in the surfaces of the best lenses ; no polish 

 is perfect ; and when a ray of light falls obliquely upon a lens, a 

 certain dissipation takes place at its surface : this is much 

 greater when two or three lenses are combined than when but 

 one is used ; of this any one will be convinced by trying the 

 experiment upon a single convex lens, and a compound object 

 glass ; in telescopical observations, the difference here noticed 

 is very conspicuous. 



When a lunar distance is measured by the method here pro- 

 posed, on account of the accurate measure obtained by the 

 divided lens micrometer, the result will serve to rate the chro- 

 nometer. Implicit reliance cannot be placed upon a chrono- 

 meter, except means be at hand by which any changes that may 

 take place in its rate of going can be discovered. When three 

 chronometers are used, the results are more certain; but on 

 account of their great expense, it is desirable to be able to cor- 

 rect one by celestial observations. The master of a merchant 

 vessel may avail himself of the advantage by using the tables; 

 and since the observations may be made almost daily, an instru- 

 ment which will not cost a fourth the price of a chronometer will 

 answer all its purposes : excellent watches, having a compensa- 

 tion balance, and going fusee, are made for less than 20/. ; and 

 if one of these be rated by a lunar observation, even but once in 

 a fortnight, it will answer every purpose of a chronometer, which 

 will cost from 801. to 1 20/. ; and if reliance be placed upon the 

 chronometer alone, not less than three must be carried out. 



There remains^ pne other case of the lunar method of the 

 greatest value, which has been too much neglected ; that is, an 

 occultation by the moon. Telescopes of sufficient power are 

 used at sea ; and Dr. Halley proved, during a St. Helena voyage, 

 that they may be readily observed in tolerable weather. Were 

 the occultations and small distances in- 

 serted in the Nautical Almanac, or other 

 Epheraeiis, the problem being but one case 

 of the plan here proposed, the most correct 

 possible measure of the longitude would 

 result. As before, let Z be the zenith, M 

 the centre of the moon, S the star : in the 

 spherical triangle Z S M, S M is directly 

 measured by the sextant, while Z S, Z M, 

 are deduced from the observed altitudes : at 

 the instant when the star touches the moon's 

 limb, as at ^ i s M analogous to S M, is 



