302 Royal Astronomical Society. 



is very small. Two massive lever-counterpoises are therefore pro- 

 vided below, which act upwards under the foot of the shaft : and if 

 these are adjusted to support the shaft when the instrument is not 

 on the lifting-forks, they will also practically nearly support it when 

 the instrument is on the lifting-forks : so that a very trifling effort 

 of the hand is then necessary to raise the shaft with the instrument. 

 This small effort is given through a winch, acting by means of 

 bevel-gearing upon a large circular nut, which works on a screw- 

 thread cut upon the shaft, and bears vertically upon one of the cross- 

 bars : and thus by an exertion which appears almost fancifully small, 

 the instrument is raised for reversion, is turned round (the telescope 

 being placed horizontally), and is deposited in its new position. Of 

 this reversing apparatus the Astronomer Royal spoke with great 

 praise. Not only is the reversion effected with a rapidity and ease 

 scarcely to be conceived, but also the counterpoises are acting in the 

 same way, and the general strains upon the instrument are almost 

 exactly the same as when it is in ordinary use. The Astronomer 

 Royal has borrowed this principle (although he has applied it in a 

 different form), in the apparatus which he has adopted for raising 

 the proposed transit-circle for the Royal Observatory, in order to 

 give opportunity for the adjustment of its collimators. 



The general form of the axis, being unencumbered by any tele- 

 scope crossing it, is evidently well adapted to the application of the 

 level at all times ; a thing which is always important, but particu- 

 larly necessary for the German instruments, which are frequently 

 counterpoised almost to the last ounce, so that there is not in them 

 the same security for the bearing of the pivots in their proper posi - 

 tion as in our instruments, in which a far greater residual weight is 

 left. 



The instrument, therefore, and its auxiliary apparatus, are most 

 admirably adapted to securing the two advantages, of easy reversion 

 and application of the level in the position of observation, which are 

 so desirable for this instrument. But these advantages, in the 

 opinion of the Astronomer Royal, are very dearly bought by an 

 entire abandonment of that mechanical firmness of connexion be- 

 tween the telescope and the axis which is obviously necessary to 

 make the observations trustworthy. The kind of firmness which is 

 required is that which retains the telescope in a position at right 

 angles to its axis ; the same, in fact, as that required for a transit- 

 instrument. We laugh at the transit-instruments of the last cen- 

 tury, in which, while great pains were taken to secure length of axis 

 between the bearings, the central connexion was left very weak ; 

 and we praise the modern transit-instruments, in which the central 

 connexion has been made successively larger and larger, and not 

 least so by the German artists ; and of which a more admirable 

 specimen cannot be cited than the Edinburgh transit-instrument, 

 made by Messrs. Repsold, the constructors of Struve's prime-vertical 

 instrument. Yet in this prime-vertical instrument, that important 

 connexion is probably very far weaker than in any transit- instrument 

 that ever was made. The whole support of the 7 -feet telescope, 



