Astronomical Methods of Observation. 133 



other. Thus, by a judicious use of the repeating circle it 

 may be employed to great advantage in all observations 

 which require a moderately sized instrument capable of 

 easy transportation. Still, however, the complex nature 

 of its construction and the involved methods of observation 

 are inherent disadvantages, which render a commodious 

 instrument of similar dimensions but more simple in prin- 

 ciple a desideratum to a numerous class of practical astro- 

 nomers. 



The only other instruments whose prices are moderate 

 and dimensions convenient are Captain Kater's circle some- 

 what enlarged, and Mr. Troughton's portable altitude and 

 azimuth circle. In these the repeating principle so much 

 recommended in Borda's, is dispensed with for the purpose 

 of securing stability and permanency of adjustment, which 

 are the main desiderata in the other. 



Though the same principle of repetition cannot be prac- 

 tised by these instruments as in that of Borda, yet the 

 observations may be repeated near the meridian with suc- 

 cess, in which the constant error arising from the imper- 

 fection of the repeated damping on Borda's plan, is thereby 

 avoided, while by means of three verniers carefully made, 

 combined with the motion of the celestial body in zenith 

 distance during the time of repetition, the errors of division 

 and pointing will, if not entirely destroyed, be greatly dimi- 

 nished — a proposition supported by uniform experience. 



In this country the use of the great mural circle perma- 

 nently fixed in the meridian is generally adhered to, and 

 by means of its size, the power of its telescope, and the 

 number of its reading microscopes, its errors are supposed 

 to be almost entirely destroyed, though the principle of 

 repetition be abandoned. Thus by the introduction of one 

 advantage, another is lost instead of attempting a union of 

 both. The smaller circles possessing the property of re- 

 peating the observations, may, therefore, approach very 

 nearly the precision of the larger, as has been proved in the 

 measurement of the French arc of the meridian compared 

 with the British trigonometrical survey. It is indeed 

 difiicult to say, whether the final results of the one or the 

 other possess the superiority, though the former was exe- 

 cuted chiefly with Borda's repeating circle of about 16 



