600 Scientific Proceedings, Royal Dublin Society. 



with its construction, it is unnecessary to point out that while in 

 some positions near the meridian, and with moderate altitudes, a 

 fair amount of accuracy of motion can be obtained, in other posi- 

 tions the direction of the driving force acts at such acute angles 

 that the greater portion tends to twist or bend some part of the 

 apparatus, instead of driving it in the required direction, which in 

 some extreme positions the mechanism often refuses altogether to 

 work. 



As the geometric principles involved actually necessitate the 

 existence of these very unmechanical conditions, there appears to 

 be no opportunity of improving the construction except by making 

 the instrument in such a manner that the work to be done shall be 

 as light as possible, while the driving portion of the instrument, 

 which is to do the work, is as strong, rigid, and powerful as 

 possible. 



In the present instrument, therefore (see Plate XIII.), the 

 mirror is mounted on a pair of trunnions, which bear lightly in Y 

 bearings ; but 90 per cent, of the weight is carried on a pair of 

 sectors which are equivalent to friction rollers of 4 ft. diameter. 

 The frame in which these trunnions are carried, which is very 

 massive and heavy, revolves on a strong steel taper spindle, and 

 has a flotation tank attached, which nearly fits into a cast-iron 

 cistern forming the base of that portion of the instrument. Mer- 

 cury is poured into this; and thus 90 per cent, of the weight of the 

 whole mirror- carrying apparatus is actually floating on mercury, 

 the result being that this frame, mirror, &c, which weighs some 

 200 lbs., can be revolved, &c, by a force of less than 2 oz. 



On the other hand, the equatorial portion, which acts as the 

 driver, is made of a strength and solidity capable of carrying effi- 

 ciently a 6-in. or 8-in. telescope, and the clockwork is of the same 

 size and strength used with the instruments for the Photographic 

 Survey, which each carry a 13-in. and 10-in. telescope. In this 

 way I hope to attain an accuracy of movement greater than is 

 obtained with instruments of the usual proportions. The clock- 

 work is supplied with the newest form of electrical control, which, 

 from recent experiments with the photographic telescopes, we 

 find we can thoroughly depend on to the one-fortieth part of 

 a second. 



This particular instrument is intended for use with a large 



