3 1 8 Astronomical Socictj/. 



But t!ie ai list has done still more; he has introduced a hand 

 on a graduated dial of the clock, by which the motion of the 

 latter can be instantly altered; so that a star may be brought 

 to any point of the field of visioTi to whicli it may suit the ob- 

 server to carry it, according as it is required to make the course 

 of the instrument go faster or slower than the motion of the 

 heavens ; and if once placed, it may be kept in that position 

 by returning the hand to its original position. The same me- 

 chanism is also used to make the motion of the instrument co- 

 incide with that of the sun and moon. 



This instrument has four eye-glasses, the least of which 

 magnifies 1 75 times, and the largest 700 times. 



M. Struve has compared the power of this telescope with 

 Shroiiter's 25 -feet reflector, by means of which that astrono- 

 mer saw (T Orionis twelve or thirteen fold ; whereas Struve 

 clearly ascertained the existence of sixteen distinct stars. 



This instrument is furnished with four annular micrometers 

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

 of the same artist. By means of these it appears that the pro- 

 bable 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/. sterling. 



There was also read a paper on " A new Zenith Micro- 

 meter ;" by Charles Babbage, Esq. F.R.S. &c. The object of 

 the inventor in this instrument is to supersede the necessity of 

 extreme accuracy in the divisions. The principle on which 

 this instrument depends may be readily comprehended by 

 imafrininn- a ijarallelogram, admitting of free motion about its 

 four angles, to be placed with two ot its sides in a horij^ontal 

 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 perpendicu- 

 lar bars of the instrument round its upper joint will not change 

 the angle which the telescope 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 hori- 

 zontal 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 onl}' remove the telescope from its position, but 

 will cause it to form a very small angle with its former direc- 

 tion. The magnitude of that angle will depend on the altera- 

 tion in the length of the arm of the parallelogram, and also 

 on the angle which that arm makes with its first direction. 

 The minutiae of the construction depend upon these conside- 

 ratipns, but cannot be rendered inteUigible without a diagram. 



The 



