Vol. tXXXIII,] PHILOSOPHICAL TRANSACTIONS. 303 



motion parallel to the equator ; and hence some person, not very learned, gave 

 it the name, machine parallactique, as if TrajaAXaxTo? and 7rafaAx«Ao? were the 

 same word. It is true that the early astronomers did use a machine called regulae 

 parallacticae, bat that was an instrument to take the altitudes of the moon, and 

 from thence to determine her parallax. Nor can I say much in favour of a ma- 

 chine of the same name, described in La Lande's Astron. vol. 2, § 2004, which 

 certainly does not do a great deal of credit to the state of the mathematical arts 

 among the French ; it however may have its convenience, as it is probably at- 

 tainable at a very small expense. The author last-mentioned speaks (§ 240g) of 

 an equatorial in his possession, made by one Vayringe in 1 737, with circles of ^ 

 or 8 inches diameter, but of what construction we are not informed ; and the 

 name of the artist is, I confess, totally new to me. An instrument also of this 

 nature, made by Megnie, for the President De Saron, is described, and seems 

 to be well imagined for a portable machine. This very amiable and ingenious 

 gentleman, Mons. De Saron, was so obliging among other civilities when I was 

 at Paris in 1775, to show me a small reflector on an equatorial stand, with some 

 wheel work to keep it constantly following a star, together with an apparatus for 

 the refraction, altitude, and azimuth, if I recollect right ; and in the year 1778 

 Mr. William Russel, a late worthy member of the r. s. showed me a small 

 instrument of the same kind, that had been made by the late Mr. Bird. 



From the preceding account, it must appear that the equatorial instruments 

 hitherto made, either from the smallness of their dimensions, or defect of their 

 constructions, were totally unfit for the accuracy of modern astronomy, where 

 an error of a few seconds only, in an observation, is all that can be admitted, to 

 entitle it to any credit.* With respect to the precision of astronomical instru- 

 ments in general, I may notice by the way, that from the time of Hipparchus 

 and Ptolemy, before and at the commencement of the Christian aera, to the age 

 of Walther and Copernicus, in the beginning of the l6th century, few observa- 

 tions can be depended on to within less than b, 8, or perhaps even 10 minutes ; 

 those of Tycho Brahe indeed, that princely promoter of astronomy, to within 1 

 minute. The errors of Hevelius's large sextant of 6 feet radius, towards the 

 middle of the last century, might amount to 15 or 20 seconds. Flamsteed's sex- 

 tant to 10 or 12 seconds ; and lastly, those of Mr. Graham's mural quadrant of 

 8 feet radius, with which Dr. Bradley made so many observations from 1 742, 

 might amount to 7 or 8 seconds. 



Having said thus much generally on the subject of this ingenious instrument, 

 and not more, I trust, than will be deemed, by every lover of this science, what 



• I must except from this remark the two large equatorial sectors made by Mr. Sisson, for Green- 

 wich Observatory ; and also an instrument of this kind, made by Mr. Ramsden, for the late General 

 Roy, and now in the possession of Mr. Aubert, whose circles are about 30 inches in diameter. — Orig. 



