and constructed hy Professor Amici. 307 



warped by the pressure of the adjusting screws, which are 

 placed behind the frame upon which they are mounted. Messrs. 

 Ludlam and Dollond have in a most ingenious manner reme- 

 died this ; but there have been mstances where mirrors sub- 

 stantiallj' mounted in brass frames, by tlie sole effect of the ex- 

 pansion of that metal from the sun's heat, have been more or 

 less warped, which is so much the more dangerous as the ob- 

 server has no means of being aware of it. Nothing of this 

 nature is to be feared from the manner in which the prisms 

 are mounted on INI. Amici's sector; they are never subject to 

 be impaired, and are exposed to no error ; they may always be 

 kept clean and neat, are easily fixed and carried, one upon the 

 sector, the other upon the index ; they are never cramped in 

 then- frames or in their motions. 



In the fourth place, the greatest and most essential of all the 

 advantages which the instrument of M. Amici has over that 

 of Hadley, is that we can make the back observation, that is 

 to say, with the horizon of the sea, diametrically opposite, 

 without adding a third prism ; because, as we have seen in the 

 description given by the inventor, we can measure with this 

 sector an angle of more than 180 deg. This advantage is really 

 of the first importance and utility not only for seamen, who 

 coast along shores, which intercept from their view the horizon 

 of the sea on that side on which the sun is, but also for astro- 

 nomers and geographers, who would use this instrument upon 

 land by the help of an artificial horizon. The difficulties at- 

 tending the rectification of mirrors for the back observation are 

 well known, which occasions their behig so rarely used, and 

 that they are even suppressed upon all sextants*. 



The astronomers, who with this instrument and the artificial 

 horizon would determine upon land the latitude of places by the 

 double meridian altitudes of the sun, cannot, particularly in low 

 latitudes and for a long time together, use the sextant. The 

 greater part of these instruments can measure only to 124 deg., 

 at least those of Troughton and Schmalkalder go only to that 

 limit. Thus at Genoa, from the 7th of May to the 8th of Au- 

 gust, we cannot with these instruments take the double meri- 

 aian altitudes of the sun. At Cairo, no observation can be made 



* On tliis head may be consulted a very important memoir contained in 

 the 74th vol. of the Gcntlenian's Magazine, and in the 14tii vol. of the 

 Naval Cin'onicle for 180."», paj^e 21 ; the title of wliich \» "A demonstrable, 

 accurate, and at all times practical method of adjusting Hadicy's sextant, 

 so as to render the back observation eijnally correct with the fore observa- 

 tion ; anil to measure an angle of loO, l(iO or 170 deg. as accurately as one 

 of .'JO, 40 or ."iO deg.,conniiunicatcd to the Astronomer Itoynl by letter dated 

 28th September, 1S03, by liie Rev, Michael Ward of Tsunwoith, Staffbrd- 

 bhirc." 



Q (] 2 fiom 



