26 Mev. Mr. Emmett on v [Jan. 



reflecting principle of half the radius ; if then its radius be nine 

 inches, it is equivalent to an instrument of 4i inches radius, 

 which is not on the reflecting principle. The greatest defect of 

 the sextant, except it be constructed on Sir I. Newton's principle, 

 which is decidedly the best, arises from the very low power of 

 the telescope which is applied to it. The aperture of the tele- 

 scope never exceeds one-half or three-fourths of an inch, and the 

 power is generally about five or seven times. Now^ it is well 

 known to all practical astronomers, that when the aperture of a 

 telescope is very much reduced, the inflected light causes great 

 indistinctness ; small as is the aperture of the telescope, half of 

 it only receives the rays which emanate from each object ; the 

 inflection which takes place at the termination of the silvered 

 part of the horizon glass, and about the cell which includes the 

 object glass of the telescope, renders the vision indistinct. Also 

 the rays proceeding from one object suffer two reflections, and 

 those from the other pass through a thick plate of glass placed 

 obliquely ; the consequence of this, and the very small aperture 

 used, is, that the field of view is so dark, that stars are observed 

 with great difficulty, and the horizon cannot often be seen in 

 the night. iMthough the telescope may magnify six times, in 

 which case an angle of 10" will be just visible, 1 know from my 

 own experience, and the testimony of others well versed in these 

 matters, that on account of the darkness of the field of view in 

 the night time, and the fringes attached to the objects viewed, 

 a measure, correct to a minute, is a very good one. 



What I have already stated will justify an attempt to render 

 the lunar method more accurate, and to furnish the masters of 

 merchant vessels with a method which shall remove all the diffi- 

 culties which at present exist in working out the observations. 



During a voyage to St. Helena, the great astronomer Dr. 

 Halley proved that a telescope of five or six feet long might be 

 easily managed in moderate weather, and that occultations of 

 the fixed stars and planets might be observed ; hence he pro- 

 posed occultations to be used for finding the longitude ; and 

 since telescopes, magnifying 20 or 30 times, are now constantly 

 used at sea, 1 do not anticipate any objection to instruments of 

 the like power ; those which Dr. H. proved might be used, 

 magnified much more. A more correct value of the longitude 

 cannot be obtained than that deduced from an occultation, and 

 it is to be regretted that they have not been more generally used, 

 and that so few have been noticed in the Nautical Almanac ; for 

 they happen so frequently, that even with the aid of a good 

 watch, the longitude might always be known to half a degree ; 

 and by them the rate of a chronometer may always be ascer- 

 tained. 



Instead of measuring the ijioon's distance from a star when 

 that distance is great, 1 propose the use of a refracting telescope, 



