60 LOCAL VARIATIONS AND VIBRATIONS OF EARTH’S SURFACE. 
Falling means that the western pier, which is the higher of the 
two, is getting lower; and, in connection with this, it is interesting 
to remark that during the past three years men have been at work 
cutting off irregularities in the cliffs on the western side of the 
hill and trimming the face to a perpendicular line ; a large quantity 
of stone, amounting to about 5,000 tons weight, has been gradually 
moved, and one would expect the removal ‘of such a mass of 
the instrument shows a fall on the west side; I think we must, 
therefore, conclude that no appreciable effect on the instrument 
been caused by the removal of the stone. The sandstone of 
which the hill j is composed possesses nes ichiataa marked strati- 
fication in beds which are nearly horizontal, but t rough these 
there are vertical joints running in ‘ies about N.N.E. .-» So that 
the rock which was cut away was separated from the part on 
which the Observatory stands by several of these joints. There is 
one other point I wish to draw your attention to in the level curves, 
of the instrument, and in many observatories such deviations are 
d as errors, ‘and corrected to the mean line or probable error ; 
hes in face of theo served changes in the earth’s surface which 
affecting the instrument for some hours only, or during the time 
the observations were made. I was very much interested by 
what Mr. Darwin says at the end of gs careful examination into 
the Gadinte on this point, viz.— enture to predict that at 
but will regard corrections derived from apetiak instruments as 
necessary to each astronomical observation.” 
Looking at the curves ——- the azimuth, you will see’ 
that there is little or no sign of any annual variation. Since the ~ 
instrument was set up, the scitatiih has gradually increased each 
year at the rate of 5” per annum for the two first years and 3” per 
annum during the four last years. The more rapid change of 
azimuth during the first years was probably due to the same cause 
_ which made the level change, but whether the gradual increase in 
the azimuth, the instrument turning towards the east, is to go on 
remains to be seen. I remember when I first jo oined the Obser- 
vatory the Rev. Mr. Scott was then watching a since te change 
in level of the same character, which went on for many months, and 
so se that he computed the Observatory would in 260 years, if 
the ra ; 1, topple over into Darlin g Harbour. the 
ities of the transit instrument foundations mf been but recently 
built, this change was perhaps not surprising: bat ceased in due 
