310. Proceedings of Philosophical Societies. [April, 



M. Slruve has compared the power of this telescope with 

 Schroeter's 25-feet reHector, by means of which that astronomer 

 saw 9- Orionis, twelve or thirteen fold ; whereas Struve clearly 

 ascertained the existence of sixteen distinct stars. 



This instrument is furnished with four annular micrometers of 

 Frauenhofer's construction, and an excellent net-micrometer of 

 the same artist. By means of these it appears that the probable 

 error in the measurement of some minute distances of 7'^ and 

 under, did not exceed the 18th part of a second. The expense 

 of this instrument was about 950/. sterhng. 



There was also read a paper on "A New Zenith Micrometer;" 

 by Charles Babbage, Esq.FRS. &c. The object of the inventor 

 in this instrument is to supersede the necessity of extreme accu- 

 racy in the divisions. The principle on which this instrument 

 depends may be readily comprehended by imagining a parallelo- 

 gram, admitting of free motion about its four angles, to be 

 placed with two of its sides in a horizontal position, and the 

 whole in a vertical plane ; and a telescope to be fixed at right 

 angles to the lower horizontal bar of this parallelogram. Here 

 every motion of one of the perpendicular bars of the instrument 

 round its upper joint will not change the angle which the tele- 

 scope makes with the meridian ; but will merely remove it into 

 a new position in which it will point to the same object in the 

 heavens. But if either of the horizontal bars of the instrument 

 be lengthened by a very small quantity, this parallelism of the 

 telescope will no longer be preserved, but any movement of the 

 upright bars round their axes will not only remove the telescope 

 from its position, but will cause it to form a very small angle 

 with its former direction. The magnitude of that angle will 

 depend ou the alteration in the length of the arm of the paralle*^ 

 loo-ram, and also on the angle which that arm makes with its 

 first direction. The minutiee of the construction depend upon 

 these considerations, but cannot be rendered intelligible without 

 a diaoram. The arc which is actually measured in the heavens 

 by means of this instrument is determined by a formula, in which 

 the sum of three arcs is taken from the semicircumference, one 

 of them resulting from the actual observation; the other two 

 from a cosine and a tangent, ascertainable by computation from 

 the theorem itself. In" an extensive use of this micrometer, 

 tables may easily be formed to facilitate the computation. 



GEOLOGICAL SOCIETY. 



Jan.2l. — A paper was concluded, entitled "On a recent 

 Formation of Freshwater Rock Marl in Scotland, with Remarks 

 on Shell Marl, and on the Analogy between the ancient and 

 modern Freshwater Formations;" by Charles Lyell, Esq. 

 Sec. GS. 



