Movements of tlie Ground. 195 



the changes of inclination since these epochs towards the east 

 and north respectively. The straight line joining an}^ two 

 points on the curve so obtained represents in magnitude and 

 direction the resultant change of inclination during the cor- 

 responding interval. The direction of the longest axis of the 

 curve is, therefore, that in which the greatest change of incli- 

 nation has taken place. 



In order to show the resultant change of inclination for 

 every day in the year, the figures would, however, have to be 

 inconveniently large. I have therefore made use of the 

 average weekly changes of inclination only ; except that the 

 average is taken for a period of eight days at the end of the 

 year of observations (Sept. 23-30), and also in the year 1884 in 

 the interval including Feb. 29. The curves so obtained for 

 the fourth to the eighth years of observation inclusive (1881- 

 86) are given in fig. 1 ; the scale being such that one inch 

 represents a change of 12" in either direction. To avoid 

 confusion the curves for different years are drawn below one 

 another instead of being superposed. 



Fig. 2 represents, on a scale ten times as great, the daily 

 changes of inclination for the period corresponding to the 

 right-hand portion of the curve for the seventh year (Dec. 24, 

 1884, to March 3, 1885). The dotted line, which represents, 

 on the same enlarged scale, the average weekly changes of in- 

 clination during the same period, illustrates the simpler and 

 average character of the latter curve. 



From the curves in fig. 1, we may conclude : — (1) That 

 the changes of inclination take place in different azimuths, 

 showing that the component movements indicated by the two 

 levels are not always in the same proportion to one another ; 



(2) that, besides the yearly periodic movement, there is 

 during the whole time illustrated a movement in a northerly 

 direction, and, from the fifth to the seventh years, a consi- 

 derable movement in an easterly direction as well ; and 



(3) that the direction of the longer axis of all the curves is, 

 on an average, about 12° S. of E, showing that tlie greatest 

 change of inclination during the year takes place appro x'lmately 

 in the direction of the average slope of the surrounding ground. 



Connexion between the Changes of Temperature and the 

 Movements of the Ground. — Let A B (fig. 3) represent a 

 section of the surface of the ground (supposed plane), P, Q 

 two points on the horizontal floor of the cellar. 



Let us suppose the external temperature to be rising. Let 

 Q G represent the isothermal surface through Q, and D E that 

 at which a given change of external temperature is just per- 

 ceptible at the end of a certain time. If Q D, P CE be vertical 



2 



