the Differences of Latitude. 357 



the axis, and taking the mean of both. The success solely 

 depends on the quality of the telescopes and the care employed 

 in the levelling of the axis. It also appears to me that those 

 astronomical amateurs who possess but indifferent instruments 

 for the measuring of angles, would thus obtain a determination 

 of their latitude with greater certainty by means of a small and 

 portable transit-instrument, even if it wei'e not more power- 

 ful than the common telescopes in the small repeating circles. 

 This method, in its application for the determination of decli- 

 nations under the supposition of the latitude being known, has 

 the advantage of being quite independent of the refraction ; 

 but it can only be employed for stars north of the equator. 



What ought to render this method still more interesting to 

 j/ou is, that your famous countryman Olaics Roiuer (who in his 

 ideas of astronomical observations and instruments has sur- 

 passed many of our moderns) employed, 120 years ago, a 

 transit instrument placed from east to west, which is described 

 in Horrebow's works, vol. iii. pp. 228 — 240, and who assigns 

 the imperfection of the instrument as the reason for its not 

 having been subsequently employed for the observation of de- 

 clinations. — The work just alluded to, which I obtained but 

 a few weeks ago, contains so many excellent things of Riimer, 

 that I am inclined to consider it as one of the most important 

 works on practical astronomy ; and I take this opportunity to 

 observe how much might have been done in the art of observ- 

 ing, even in Romer's time, if the path he took had not been 

 again abandoned. 



\Note hi) the Editor. — The work here alluded to by M. Bessel 

 is entitled " Petri Horrebowii, Opera Mathematico-physica." 

 Hauniae, IT^O, 3 vols, quarto. The third volume contains 

 the following treatise : " Basis Astronomiae, sive Astrono- 

 jnia? pars mechanica, in qua describuntur observatoria, atque 

 instrumenta astronomica Rocmeriana Danica; simulque eo- 

 rundem Usus, sive Methodi observandi Rocmeriana;." The 

 xviii"' chapter of this treatise is entitled " De instrumento 

 iTlquinoctiorum Rcemeri :" and this is the instrument alluded 

 to by M. Bessel. 



But the use of a special instrument, for the purposes here 

 alluded to, is now superseded by the intrt)duction of the alti- 

 tude and azinuith instrument ; which seems peculiarly adapted 

 for observations of this kind. 



If the circle be placed exactly east and west, we shall have 

 cot ip = cot 8 . cos ^ (T' — T) 

 where (T' — T) denotes the correct interval of sideical time 

 elapseil between tlie two observations, expressed in degrees, &c. 

 Bill, if (T' — T) be taken in mean solar time, we must multi- 

 ply it by 1*0027375> in order to reduce it to sidereal time.] 



LVII. yl De. 



