Mr Galbraith on a New Pocket-Box Circle. 227 



ment of astronomy were derived from the labours of those astro- 

 nomers to whom they were entrusted. 



The first meridian transit circle, so far as we know, was that 

 of Horrebow, constructed about the year 1735. The next, 

 that of Mr Francis Wollaston in 1793, made by the late Mr 

 Gary, though Ramsden had made the Palermo altitude and 

 azimuth circle in 1789, and the Dublin circle some years after- 

 wards, both of which have rendered essential services to astro- 

 nomy. These were all of great size, and fitted for fixed obser- 

 vatories only. A smaller, and more portable circle, was still 

 required to supply the wants of amateur astronomers and scien- 

 tific travellers. Mayer of Gottingen first contrived an instru- 

 ment by which he could diminish the errors arising from bad 

 dividing, by the principle of repeating the measurement of an 

 angle over the whole, or, at least, a great part of the circum- 

 ference. Shortly afterwards, Borda invented his celebrated re- 

 peating astronomical circle and his repeating reflecting circle, 

 of which so much use has been made on the Continent, and to 

 which such extravagant eulogiums have been paid by the Con- 

 tinental observers. Undoubtedly, the results obtained by 

 means of these instruments, particularly the former) are in many 

 instances remarkable for their accuracy, considering their mode- 

 rate dimensions. In fact, all the operations, astronomical and 

 geodetical, required in the determination of the French arc of 

 the meridian, were made by Borda's repeating circles of about 

 eight inches radius ; while in the British trigonometrical sur- 

 vey, the astronomical observations were chiefly taken by a 

 zenith sector of eight feet radius, and the terrestrial by a theo- 

 dolite of eighteen inches radius ; and it is at this moment diffi- 

 cult to say, notwithstanding the disparity of the dimensions of 

 these two classes of instruments, which of those two grand ope- 

 rations have been executed with the greater accuracy. Instru- 

 ments of moderate dimensions, capable of repeating the obser- 

 vations frequently in a short space of time, seem by these, and 

 other instances that might be produced, to approach the accu- 

 racy of the larger instruments much more nearly than might 

 have been anticipated. 



A reflecting circle was constructed by Bird for Admiral 

 Campbell, chiefly from the description of one invented by 



