132 PHILOSOPHICAL TRANSACTIONS. [aNNO 1737- 



M. Picart, for his first operation, got an arch of a circle made of 18 degrees 

 and of 10 feet radius, with which he thought himseU' sure within 2 or 3 seconds: 

 and no other instrument was used in the chief observations for the meridian of 

 Paris. The astronomers who are gone to America, carried with them an in- 

 strument of 12 feet radius, and of a portion of a circle of 30 degrees. But 

 those come to Sweden, contented themselves with a portion of a circle of 54- 

 degrees, and 9 feet radius: but this instrument, made by Mr. George Graham, 

 a very able English mechanician, is by its construction so exact, that the 

 astronomers who have used it, think, themselves certain to 2 seconds. The one 

 wanted for the observations in Russia, ought to be made by the same artist, 

 and of the same construction. 



It is with such an instrument that Mr. Bradley, a celebrated English astro- 

 nomer, has discovered, in the meridian altitudes of some fixed stars, certain 

 constant and annual variations, which do not proceed either from the variation 

 of the refractions, or from the parallax of these stars, or from any nutation or 

 wavering of the earth's axis ; but which he accounts for by the successive mo- 

 tion of light. Whatever be the cause of these variations, as they may possibly 

 happen in the space of time requisite to be spent in making the observations for 

 the meridian, or in passing from one end of the meridian to- the other; it is 

 necessary, with the same instrument, or such another, that is of pretty near 

 the same exactness, to examine the variations of the stars made use of: it 

 would therefore be of considerable advantage, not only for the observations of 

 the measurement of the earth, but also for all the other principal researches in 

 astronomy, to have orders given for procuring two mural quadrants of Mr. 

 Graham's make, and of the same construction, as already specified; for which 

 there are walls already raised at the imperial observatory, in the plane of the 

 meridian. With these two quadrants, which might be of 7 feet radius, and the 

 moveable telescope 9 or 10 feet long, we should be in a condition to make ob- 

 servations of the utmost accuracy, such as the present state of astronomy 

 requires. 



Besides these instruments now mentioned, which are of absolute necessity to 

 a solid establishment of astronomy and geography in this country, there are still 

 some other smaller instruments, that may be of great use in the operations, or 

 may serve to make other curious and useful observations at the same time, that 

 those for the measurement of the earth are making. 



When the sides of the triangles, taken for measuring the earth, terminate at 

 very elevated places, as on the tops of the highest mountains, it is necessary to 

 reduce these triangles to what they would be, had they been observed in hori. 

 zontal planes on a level with the sea. For this purpose, we must know the 



