BY. Q. R. SMALLEY, ESQ. 345 



ture or actual periodic changes in the position of the ground 

 upon which the instrument rests. 



Now it is hardly to be conceived that tha expansion and con- 

 traction of the different parts of this complicated instrument 

 the transit circle should follow such regular laws as those which 

 appear to obtain in the case before us. And even if it were so, 

 we should expect that the extremes of azimuthal deviation would 

 correspond to the extremes of external temperature but this is 

 contrary to observation. Can we then do otherwise than conclude 

 that these remarkable periodic variations are due to an actual 

 twisting if I may use the expression of the hill or ground 

 upon which the Observatory is established ? 



It will naturally be enquired whether such a phenomenon is 

 confined to elevated spots. I am inclined to think it is. I think 

 I have traced it at Sydney it has been suspected at Edinburgh 

 and Trivandrum and I have endeavoured to verify it in the case 

 of Greenwich : all these are elevations of some magnitude. On 

 the other hand, I have carefully examined the observations made 

 at the Observatories of Oxford and Cambridge both of which 

 are situated on comparatively low and gently rising eleva- 

 tions, and I find, not only that the azimuthal changes are 

 very small, but no regular law of periodicity can be detected. 



In order to contrast the irregularities which arise from instru- 

 mental or accidental causes with those which appear to follow 

 some law of nature, let me again direct your attention to the first 

 diagram in which every observation of azimuth is exhibited. 

 You will perceive two very remarkable and sudden deflections 

 amongst several others one on the 15th of April, 1859, and 

 the other on the 20th of September in the same year ; yet on 

 both these occasions I find recorded in the Greenwich observa- 

 tions that the transit circle had been raised from its bearings on 

 those days, and it was suspected that it had not quite recovered 

 its natural position at the time the observations for azimuth 

 were made. Compare these descrepancies with the general 

 symmetry exhibited by the four curves, and we cannot help feeling 

 that the laws of nature assert their superiority, notwithstanding 

 accidental irregularities which are easily detected. 



In order to connect, as far as possible, these changes which 



