Royal Astronomical Society, 303 



upon the firmness of which support the value of the observations 

 entirely depends, is a single perforated pivot, 4 inches in diameter, 

 on one side of the telescope. It is true that the telescope is sus- 

 tained, as regards the strain of its own weight upon the small pivot, 

 by an internal concealed counterpoise (for no one who has mastered 

 all the external counterpoises of a German instrument is therefore 

 to suppose that he has possessed himself of all the applications of 

 that principle in the interior of the instrument), whose fulcrum is in 

 the perforated pivot, near the telescope, and whose weight is within 

 the hollow case at the other end of the axis, which appears to the 

 eye like a large counterpoise connected with the external axis. But 

 this counterpoise, while it delivers the pivot from the ordinary strains 

 to which it would be exposed from the weight of the telescope, does 

 in no degree diminish the effect of what, perhaps, are really more 

 formidable, the accidental strains produced by pressures on the ends 

 of the telescope, or other accidental forces, or irregularities of forces, 

 not taken into account in the construction of the instrument. For 

 instance, if the Y's were slightly irregular, so that the principal 

 bearing of the pivots on the northern Y was an inch nearer to the 

 face of the pier than that on the southern Y (a thing which it would 

 be nearly impossible to discover by examination), the difference in 

 the bend of the axis in the two positions of the instrument would 

 probably be so considerable that every result would be worthless. 



In the opinion of the Astronomer Royal, the asserted consistency 

 of results hitherto obtained with this instrument proves nothing. 

 Although discordance proves the existence of some fault, accordance 

 does not negative the existence of very great faults. The Astronomer 

 Royal cited the expression of Bouguer, who, after much painful ex- 

 perience in the construction of zenith sectors, in different forms, for 

 the measure of the Peruvian arc, came at last to the conclusion that 

 no agreement of results proved their truth, unless the logical cor- 

 rectness of construction of the instrument gave reason il priori for 

 believing that the results would be good. 



One defect to which this instrument is liable was pointed out by 

 M. Struve himself to the Astronomer Royal. It cannot be assumed 

 that the temperatures of the external faces of the two piers are the 

 same ; and if they be not, the effects of their radiation upon the te- 

 lescope-tube must be different*. • 



The Astronomer Royal then remarked, that though perhaps the 

 form of the ordinary transit-instrument would not give in their full 

 extent the same facilities, yet the great importance of securing the 

 admirable firmness and excellent connexions of the transit-instrument 

 made it desirable for us to attempt to unite with them, as far as 

 possible, the peculiar conveniences of Struve's instrument. In 



* The prime vertical instrument of M. Struve, constructed by Repsohi, 

 is fully figured in plates 32, 33 of M. Struve's magnificent work, De- 

 scription de P Observatoire de Poidkova, Saint Petersbourg, 1845. Drawings 

 of the Greenwich zenith tube, and models of Struve's prime vertical instru- 

 ment and of the reflex zenith tube, may be seen at the apartments of the 

 Rojal Astronomical Society. 



