342 AZIMTJTHAL CHANGES OF HILLS, 



I have already observed that the transit circle is of the 

 hightest order. It was erected in 1850, and its entire weight 

 is, in round numbers, about 17 cwt., with its axis supported by 

 two granite piers of immense strength and thickness, sunk 

 several feet below the surface. 



I have dwelt upon these details because it is necessary to 

 premise that the observations upon which the present enquiry is 

 founded, have been made under the most favourable circum- 

 stances, and with the greatest possible care and accuracy. 



As pictures give a clearer and more impressive view of events 

 than any verbal or written description, so I think curves 

 represent more conspicuously than figures, such periodic changes 

 as those which I am about to discuss. 



In these diagrams, one side of each of the squares represents 

 one second of arc for azimuthal deviation the blank line being 

 the meridian and another side perpendicular to it represents 

 an interval of three days. In Fig. I., every observation for the 

 determination of error of azimuth is set down, and the points 

 corresponding to such observations jointed by lines. Hence the 

 curves thus described represent the actual deviations to the east 

 or west of the instrument itself, or of the ground upon which the 

 instrument stands. In Fig. II., the curves are described " libera 

 manu " through the mean positions of the deviations. Now, on 

 examining these diagrams, we cannot fail to be struck with the 

 remarkable similarity of the general outline of the curves. You 

 will perceive that in each of the four years they appear to indicate 

 a maximum elevation towards the east about the vernal equinox, 

 and a minimum deviation towards the west, about the autumnal 

 equinox. Carried over four consecutive years, we can hardly suppose 

 such a phenomenon to be accidental. No doubt you will remark 

 many irregularities in these curves, but I am rather glad to see them 

 than otherwise, because in spite of errors of observation and mecha- 

 nical defects, it brings out the laws of nature in greater relief. 



I have already endeavoured to explain how these azimuthal 

 changes are obtained, and it now remains for us to examine how 

 this apparent periodic regularity may be accounted for. Astro- 

 nomical causes are out of the question, so that I know of no 

 other alternatives, except instrumental changes due to tempera- 



