532 Royal Astronomical Society, 



•who can command a rock foundation, should make use of their good 

 fortune ; and that those who cannot, would look carefully to the 

 possible effects of moisture, which are probably more extensive, and 

 vary more rapidly, than those of temperature. 



The observatory of Edinburgh is placed on the Calton Hill. This 

 is chiefly of a porphyritic formation. The apex w^as blasted away 

 to obtain a level area, on which the observatory was erected. The 

 site of each pier was cut away until a sound part of the rock was 

 arrived at (it was not necessary to go deeper for this purpose than 

 six or nine inches), when the exact size of the foundation was at 

 once marked out and the space carefully levelled. The foundation 

 stone was also carefully smoothed, and then laid in its place with 

 milk of lime. As the foundation and stone were both rather hollow, 

 except for three inches at the outer edge, which was polished, the 

 fitting was very perfect. There are no vertical joints, and each stone 

 was laid in the same manner as the foundation stone. As one of 

 the principal thoroughfares of Edinburgh runs about 100 feet below, 

 and only 300 feet distant from, the observatory, tremors were con- 

 fidently predicted by the alarmists. Professor Henderson, however, 

 found none, nor any interruption to his observations in mercury. 

 Professor Smyth adds that he finds no annoyance from the railroad 

 about 300 feet below, and at a horizontal distance of 500 feet. 



So far the observatory founded on a rock came out victoriously 

 from its ordeal, but Professor Henderson, in the course of his work, 

 found a well-marked annual variation of the level of the transit, 

 which he attributed to the expansion of the rock. This variation 

 seemed so intimately connected with temperature that he latterly 

 took his factor for level correction from the thermometer, having 

 found a constant agreement between this and the indications of the 

 spirit level. The maximum of this change amounted to between 

 0^-2 and 0**3 in the value of the level factor, and the variations were 

 tolerably regular. 



On computing the azimuthal factors for 1841, Professor Smyth 



is more felt on solid than on loose foundations. In a standard observatory, 

 where observations are made principally in the meridian, tremor scarcely 

 affects the accuracy of observation at all, unless it is so excessive as to 

 change the position of the microscopes, piers, &c. Now this is obviously 

 the least likely to happen when the foundation is on rock ; the tremors are 

 propagated through the substance, without in any respect altering its form. 

 Sudden and discontinuous changes, which obey no law, are those only which 

 are to be feared in a well-directed observatory. Tremor is chief!}' objec- 

 tionable as disturbing the mercurial horizon, which, however, is now mostly 

 used as a verification, not as the ordinary mode of observing; and when 

 this inconvenience only occurs occasionally, it can generally be avoided or 

 palliated by a little contrivance or foresight. Unless the adjustments are 

 kept in a fluctuating and uncertain state by occasional small oscillations 

 (and we believe no careful experiments have been directed to this point), 

 they are minor evils. The experience of the Oxford and of the Edinburgh 

 Observatory is, so far as it goes, conclusive against any danger from mo- 

 derate exposiu'e to tremors in a well-founded and well-managed observa 

 tory.— S. 



