138 , SCIENTIFIC NO'l'KS. 



Anotl ior change is that all modern meridian instruments have fl 

 circle on each end of the axis — one being fixed, the other free to 

 move when wanted for testing the graduations of the fixed one. 

 Each has its own set of four microscopes, supported not on the 

 stone piers as of old, but on a metal circle attached to the Ys, or 

 supports of the instrument. Graduated circles have always been 

 made as an open framework. At first they were built up of 

 several pieces, circle and radial arms being screwed together. 

 Next, to ensure more equal temperature effects, circles were cast 

 in one piece, but still with open spaces, one arm might therefore 

 be affected by a change of temperature which did not reach the 

 others. Now, in the new transit circle which I had the pleasure 

 of ordering for our Observatory when in England, the circles are 

 for the first time solid, so that such partial effects can no longer 

 take place. Mr. Sinmis considers that this will give the best 

 results, because it is the form used in the dividing engine with 

 which such splendid results have always been obtained. In 

 most modern transit circles also, errors of from one second to two 

 seconds of arc have been found, and at last traced to a yielding of 

 the tube where it is made fast to the axis, partly owing to weak- 

 ness, and partly to the excessive weight placed at the ends of the 

 tube. Both these defects have been cured in our new instrument, 

 and I hope it will prove one of the most perfect ever constructed. 

 One other point with regard to the transit instrument. It will be 

 within the memory of some here present that, some years since, my 

 predecessor (Mr. Smalley) read a paper before this Society on 

 " azimuthal changes in the Sydney transit circle," similar to what 

 had been found in several other observatories, and tried to account 

 for this as a result of temperature on the rocks under the Observa- 

 tory. A similar result had been found at Edinburgh, and the 

 circumstances were such as led the Astronomer Royal for Scotland 

 (Piazzi Smythe) to suspect it was all to be attributed to the 

 effect of the temperature on the stone piers which supported the 

 instrument, even though they Avere of the same stone, size, and 

 shape. The changes he found were, as in our case, in level as well 

 as in azimuth, that is, the observations showed that at times 

 one pier got longer than the other, and that at other times one 

 or both piers twisted the instrument out of the meridian by 

 leaning over to north or south. To prove it was an effect of tem- 

 perature, he put a small hand oil-lamp close to the south side 

 of the pier, whilst the telescope was directed on a mark, and at 

 once he saw an effect in azimuth ; the pier had expanded on that 

 side, and leaned over to the north. Removing the lamp, he allowed 

 the pier to recover its position, and then applied lamps on both 

 north and south sides of the same pier, and, watching the level, 

 saw the effect of the lamps ; for the pier expanded gradually, 

 changing the level of the instrument. One of the first things that 



