Small Vertical Movements of a Stone on the Ground. 253 



" On the Small Vertical Movements of a Stone laid on the Surface 

 of the Ground." By Horace Darwin. Communicated by 

 Clement Eeid, F.RS. Eeceived April 17, — Eead May 2, 

 1901. 



' In my father's book on Vegetable Mould and Earthworms an esti 

 mate is given of the rate at which stones placed on the surface of the 

 soil are buried by the action of earthworms. The estimate is rough, 

 and as far as I know no attempt has been made to detect such move- 

 ments when small, or to determine them accurately when they are 

 large. 



The experiments described in this paper were undertaken originally 

 to measure accurately the downward movement of a stone caused by 

 earthworms. The upward and downward movements due to varying 

 moisture of the soil and to frost were found to be much larger than 

 was expected. These movements, interesting in themselves, increase 

 the difficulty of accurately determining the movement due to the 

 action of earthworms.* 



The experiment was begun on September 5, 1877, and the position 

 selected is in a nearly level field which had probably been pasture for 

 considerably more than fifty years. It is to the south of my father's 

 house at Down, close to some railings separating the field from the 

 lawn and under a large Spanish chestnut tree. He approved at the 

 time of the selection of this position ; at a later date he considered a 

 mistake had been made, as he thought there were fewer worms under 

 trees.! 



It was necessary to have a fixed point from which the displacement 

 might be measured ; this was managed in the following way : — An 

 iron rod was driven into the ground by means of a heavy hammer ; it 

 was then removed, and a copper rod, slightly larger (22 mm. in 

 diameter), was driven into the hole ; the bottom of the rod was about 

 2*63 metres from the surface. The top of this rod is the point from 

 which all measurements were taken. 



A circular stone about 460 mm. in diameter and about 57 mm. thick, 

 weighing about 23 kilos., was placed on the ground with the rod pro- 

 jecting through a hole in its centre. A brass cylinder, slightly smaller 

 than the hole in the stone, had previously been firmly fixed in the 

 hole by running in melted lead. The brass cylinder had three pro- 

 jecting pieces at its top ; three symmetrical radial right-angle grooves 

 were cut, one in each of these projecting pieces. This gave the usual 



* See ' Vegetable Mould and Earthworm?,' by C. Darwin, 1883, p. 121, where a 

 short preliminary account of the experiment is given. 



f Ibid., p. 146. In Knowle Park, under beech trees, worm eastings were almost 

 wholly absent. 



