Zach on Repeating Circles. 419 



sectors, and drag them over mountains and valleys. It is only in 

 Entrland that a school-master can buy an instrument which is 

 too dear for a sovereign*. 



As we before said, every thing in its place. When 

 M. RUppell prepared to leave us for his travels in Africa, we 

 did not advise him to carry a repeating instrument, either 

 great or small, for the same reason that we advised him not 

 to carry a chronometer of the best quality, or of great value, 

 for the'purpose of observing longitudes. We explained our 

 reasons for this advice in the first volume (p. 514) of Corre- 

 spondance above mentioned. If M. lliippell had wished to make 

 his observations with a repeating circle, however small, he 

 would rarely have found in the whole course of his travels a 

 convenient, " solid and secure spot, sheltered from curiosity, 

 from suspicion, or even from danger. He would have wanted 

 an assistant, indeed he would have been wholly dependent upon 

 one, to make his observations and to adjust the level of the 

 circle. With a sextant of reflexion there are none of these 

 difficulties. With such an instrument, with an artificial ho- 

 rizon, which places and keeps itself level, and with a chrono- 

 meter in his pocket, he may make an observation alone m 

 concealment, in any spot, and in the smallest opening through 

 which the sun shines. These are instruments which he may 

 put in his pocket, conceal, and carry in his portmanteau. In 

 the countries traversed by M. Riippell a question of seconds 

 is of no importance. There, where the latitudes and longi- 

 tudes are quite unknown or incorrect by many degrees, a mi- 

 nute is as much or even more than a second with us. A sex- 

 tant of reflexion does not so easily get out of order as a re- 

 peating circle ; it is more easy to handle ; one may rectify it 

 every moment, find the error of collimation at every observa- 

 tion, use it upon sea and upon land, for the heavens and for 

 the earth, determine the seasons, latitudes, longitudes, azi- 

 muths, and terrestrial angles, with a precision beyond what 

 is actually necessary or what can be reasonably desired. 1 he 

 sextant of reflexion was therefore in this instance preferable 

 to repeating circles, and even to all non-repeating mstruments, 

 because it teas here in its place. 



People had gone the length of saying that repeating instru- 

 ments had arrested the progress of astronomy, and had pre- 

 vented a great number of useful observations which would 

 otherwise have been made, since those made with instruments 

 of this description demanded a great expense ol timcf, which 



miglit 



• Tliis anccilote is well known in London. 



-I It would be possible to dispute this expense of tunc, and to show that 



' ' 3 G 2 '* 



