of a Stone laid on the Surface of the Ground. 259 



in a horizontal direction, and their vertical distance apart is 4/5 of the 

 actual displacement of the stone, the numbers on the scale representing 

 mms. This curve is marked " Winter." There were no winter readings 

 after 1886. The Summer curve is made in a similar manner ; the dates 

 of the observations are more irregular : the corresponding points, 

 however, are equally spaced in a horizontal direction. 



The measurements from which the curve is constructed are as 

 follows • the second column gives the position of the stone measured 

 in mm. : — 





mm. 





mm. 





mm. 



1878, Jan. 26 . 



. .. 30 91. 



.July 7 .. 



. 24-50 



1887, Aug. 21 . . 



. . 6-50 



1879, „ 3 . 



. .. 26-92. 



. „ 10 .. 



. 26-34 



1888, Sept, 20 .. 



.. 10-34 



1880, „ 11 . 



. .. 25-59. 



. „ 12 .. 



. 22-24 



1889, „ 17 .. 



.. 7-53 



1881, „ 9 . 



. .. 22-28. 



. „ 29 .. 



. 15-84 



1890, ,. 24 .. 



.. 8-16 



1882, „ 9 . 



. . . 20 -42 . 



. „ 10 .. 



. Aug. 1 



. 17*61 



1891, Aug. 6 . . 



.. 8-90 



1883, Apr. 3 . 



. .. 17-82. 



. 15-27 



1892. Sept. 6 .. 



. . 7-72 



1884, no winter 



reading. 



. Sept. 14 . . . 



. 11-38 



1893, Aug. 2 . 



.. 4-03 



1885, 





. July 19 .. . 



. 11-02 



1894, Aug. 24 . 



. . 6-86 



1886, Mar. 1 . 



. ..13 -13. 



. No summer 



reading. 



1895, Sept. 17 . 



. . 2-50 









1896, Aug. 2 . . 



.. 3-14 



The stone was accidentally removed and no readings were taken 

 after 1896. 



If we take the winter readings, we find that the stone sank 

 17 '8 mm. in the eight years from January 1878 to March 1886, or at 

 the average rate of 2'22 mm. per year, rather less than 1 inch in 

 ten years. My father found* that small objects left on the surface of 

 a field were buried 2:2 inches in ten years. This result is obtained 

 from observations in a field near the stone. The large stone sank 

 more slowly, a result we should expect. 



The curve shows that the rate of sinking was greater at the 

 beginning than at the end ; this is probably due to the decaying of 

 the grass ; the turf was not removed, the stone resting directly on it. 



The third curve, marked " Eain " on this diagram, roughly indicates 

 the dampness of the ground. The ordinates of the curve are propor- 

 tional to the rainfall at Greenwich Observatory during the twenty days 

 before the date of the summer reading. The curve is only a very 

 rough indication of the dampness of the soil, as no account is taken of 

 the rainfall for a longer period than twenty days before the observation, 

 and neither is the evaporation during this period allowed for. The 

 rainfall at Down also is assumed to be the same as at Greenwich, 

 although they are 10J miles apart, and Down is 569 ft. above Ordnance 

 datum, and Greenwich is 155. 



The summer curve is far more irregular than the winter curve ; this 



* ' Vegetable Mould,' 1883, p. 142, 



