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tracting, and giving and limiting with every change of temperature 

 or application of small accidental pressures. All this takes place, it 

 is true, within limits which are perfectly inapplicable to ordinary 

 observation, but are of the utmost importance to be attended to in 

 astronomical inquiries. And so certain is it that changes and dis- 

 tances must exist in some shape and some form in every case, that 

 if any one observer was hardily to declare that his telescopes kept 

 their adjustments perfectly, or had no error, the statement would 

 only be looked on by astronomers as proving that his observations 

 were very rough and inaccurate. 



The most prejudicial form in which the effects of instability can 

 manifest themselves, is in any irregular motion of the stands whereon 

 the instruments rests. 



This is usually guarded against by constructing these stands in 

 the shape of large and heavy blocks of masonry, the heavier the 

 better. But even when the greatest practicable size has been 

 reached, perfect immunity from disturbing influences is not obtained. 

 This was signally experienced at Greenwich some years ago, when a 

 telescope was firmly built into a large stone pier, with the view of 

 making such very exact observations of a certain star, as to be able 

 to ascertain its annual parallax. But long before the year was 

 elapsed, it was found that the measures were absolutely vitiated, by 

 the irresistible swelling of the hiU from rain, and the consequent 

 heaving up of one end of the pier. 



Experience therefore drew the rule, that in addition to the ut- 

 most security which a large mass can give to the pier, it is proper to 

 introduce some principle of reversal in the instrument placed upon 

 it. For with such a method, the exact state of adjustment of the 

 whole can be ascertained for any instant. Then it will probably 

 be found that the structure, the permanence of whose position could 

 not be depended on for a year, may be relied on from day to day, if 

 not implicitly, at least to within far less than the limits of the pro- 

 bable error of observation. 



In the Edinburgh Observatory both these principles have been 

 long since introduced, and have lately been carried further towards 

 perfection. 



The stone piers, for instance, which were erected by our respected 

 member Mr Jardine, are models of excellent masonry, composed of 

 peculiarly dense material, in the largest available blocks ; and what 

 is more important, they are founded on the hard porphyry rocks of 



