232 



mechanically. He therefore adopted the very proper plan of leaving 

 the adjusting screws untouched, but of measuring the amount of error 

 each day, and calculating the effect thereof on the observations. 



Still the adjustable Y could not be so firm as a plain block ; and 

 being at last pretty plainly convicted of producing the bad effects al- 

 ready described, a necessity came for introducing new and firmer 

 bearings. I can now describe the mode in which this was effected, 

 and the astronomical results which have followed. 1 applied first to 

 the German maker of the instrument, but found him far too fearful 

 of leaving the old beaten paths of instrument-making to attempt any 

 improvement. Next, therefore, I applied to Mr John Adie of this 

 city, and am happy to say that he carried out my designs in a per- 

 fectly satisfactory manner, and so caused the Edinburgh transit to 

 be the first in which this signal improvement has been made ; for its 

 advantage is now recognised, and has been adopted elsewhere. 



The new Ys are now large blocks of cast-iron, of the whole area 

 of the top of the pier, and weighing as many hundredweights as the 

 old Ys did single pounds. They have, moreover, no adjustments ; 

 but the notches in which the pivots of the instrument rest were 

 filed, by repeated trials, to within a certain small quantity of the 

 truth, and have since only been subjected to examinations for the 

 quantity of error. The result, now tested by many years, has been 

 highly satisfactory. For, firstly, they have never been so far out as 

 to I'equire a second filing, or to be out of the limits of convenient 

 calculation ; and, secondly, what small amount of variation of posi- 

 tion they have been found liable to, has been almost entirely the 

 slow and regular expansion of the piers already alluded to. There 

 has been certainly a difference in the amount of wear of either Y ; 

 but this has been exceedingly small, and has very regularly increased 

 with the time, while the large anomalous and irregular fluctuations, 

 which were the dangerous features of former years, seem to be 

 effectually removed. Even when labouring under this drawback, 

 the Edinburgh observations, though not all that they might have 

 been, were at least equal in accuracy to those of any other observa- 

 tory ; so that I trust that they will still, through this alteration, be 

 enabled to keep up their comparative character, whatever improve- 

 ments may have been made elsewhere. 



As a specimen of the increased regularity now of the march of the 

 instrument in its annual temperature movement, I subjoin the ob- 

 served errors of similar periods of the years 1841 and 1851 : — 



