ZENITH. 



Befides thefe obferrations.various others were taken of different ftars in Draco, Cygnus, Urfa Major, Hercules, Perfeus, 

 and Auriga from an average of all which the final refults were obtained ; but before the true or mean zeaith-diftances 

 can be exaaiv known, the abpartnt meafures muft be correfted by certain equations for aberration, nutation, femt-annual 

 folar equation, precejfion, and refraHhn ; the Tables proper for which are chiefly given under our article Declination-. 

 It was deemed fufficient for our purpofein tliis place to confine ourfelves to the confideration of the obfervations made on 

 y Draconis alone, which being a ftar of the fecond magnitude, and very near the zenith of London, and of the fouthern 

 parts of England,' was confidered as the beft objea- We will therefore fubjoin the redudion of the obfervations made on 

 this ftar by way of iUuftrating their application in pradical aftronomy. 



Redudlion of the Obfervations contained in Table I. 



When the reduftions are thus made for the obfervations 

 taken at the other places, the zenith-diftances and corre- 

 fponding latitudes will ftand thus, 1:1%. 



In obtaining the latitudes of the two lad places, the decli- 

 nation of y Draconis is diminifhed 3" to bring it to the year 

 1806, and in all the cafes where S is annexed to the zenith- 

 diftance, it is added to the declination. The reduftions are 

 made to the firil of January of the refpeftive years, and the 

 latitudes come out very nearly the fame as thofe determined 

 from terreftrial meafurement, and alfo from an average of all 

 the obferved ftars. 



" From the obfervations made at the ftation in Delamere 

 foreft with the zenith-feftor in 1806, combined with thofe 

 at Dunnofe, 1802, taken with the fame inftrument," fays the 

 author of the Survey, " it is found, that the difference in lati- 

 tude of thofe is 2° 36' 1 2". 2 (by "/ Draconis alone in our exam- 

 ples 2° 36' ii".73), making a difference of i" between the cal- 

 culated and obferved amplitudes, which, fettingafide the con- 

 fideration of the fpherical figure of the earth, is at the rate of 

 •^ths of a fecond in one degree. Perhaps, imderthe confi- 

 deration of each meridianal hne being obtained independently 

 of the other, and admitting that neither of them can be 

 meafured with perfedl accuracy, together with the chances 

 of the amplitudes being in fome fmall degree either in excefs 



or defeft, we may confider the refult as fufficiently confift- 

 ent and fatisfaftory, and may take 60823 fathoms, in lati- 

 tude 52° 34', or the centre of England, as the length of 

 l°." Trigonometrical Survey, vol. iii. p. 332, &c. 



ZiEynm-Micromeler is an inftrument of very recent date, 

 and is fcarcely yet known to the generality of aftronomers. 

 It differs from the zenith-feftor in this refpeft ; that the 

 meafures taken with it are all taken within the tube of the 

 telefcope, whereas thofe taken by the zenith-feftor are all 

 external : confequently the range of the zenith-microme- 

 ter's fcale is confined to the extent comprifed within the 

 field of view, which will always be inverfely as the magni- 

 fying power of the glaffes ufed ; hence the greater the 

 power of the telefcope, the fmaller the number of ftars that 

 will pafs within the range of the micrometer-fcrew ; but 

 then correfponding accuracy may be expefted from the 

 great powers and delicate conftruction of modern micro- 

 meters ; and what is wanting in the extent of the fcale will 

 be made up by fuperior precifion. The telefcope, which 

 conftitutes the bafis of the zenith-micrometer, maybe either 

 of the reflefting or refraiting conftruftion ; and within a 

 ftiort fpace of time one of each defcription has been con- 

 ftrufted ; the former by Troughton, for the Greenwich ob- 

 fervatory, and the latter by DoUond, for the ufe of thofe 

 geodatfic commifConers, who are employed by the Englifh 

 government to afcertain the proper line of demarcation 

 acrofs the American lakes. We will give a ftiort account 

 of each of thefe inftruments, fuch as will enable our readers 

 to form an opinion of their refpeftive merits, as prototypes 

 for future imitation. 



Zenith- Micrometer by Troughton — It is probable that the 



firft 



