260 Small Vertical Movements of a Stone on the Ground. 



no doubt is due to the greater variation in the dampness of the soil in 

 summer than in winter. The rain-curve and stone-curve roughly 

 follow each other. In 1888, however, the stone rises and the rain- 

 curve shows very little rain for the twenty days before September 20, 

 the date of this observation. During June, July, and August a great 

 amount of rain fell; and although there was very little rain from 

 September 1 to 20, the ground was probably damper than the rain- 

 curve indicates. At Hayes, 3 J miles from Down, the rainfall on these 

 days was greater than at Greenwich, but still very small. 



If the points marked A and B are joined by a straight line, it will 

 roughly represent the mean movement during the first nine years of 

 the experiment. These points were selected so that the line joining 

 them appeared to represent the mean movement to the best of my 

 judgment. In the same manner the points C and D were selected, so 

 that the line joining them represented the mean movement of the last 

 nine years of the experiment. The movements deduced by this 

 method are 2*3 mm. per year for the first nine years, and 0'36 mm. 

 the last nine years. The slow movements for the latter period are 

 surprising. The movement given above and obtained from the winter 

 curve is 2*22 mm. per year. 



Fig. 4. 



5 10 26 % 23 2 19 



J an. Feb. Mar. 



1879 



During the last five years the rainfall on the twenty days before 

 each observation was distinctly above the average ; it was 2 '09 inches, 

 and the average for these twenty days during the whole experiment is 



