of a Stone laid on the Surface of the Ground. 255 



and the micrometer screw was used for the measurement. G and H 

 are the tops of the iron and copper rods ; the micrometer screw is 

 turned till its lower end K just touches one of the rods ; the upper end 

 of the screw is not used at all. The stand and micrometer were kept 

 indoors till wanted. 



Fig. 1. 



The method of reading was as follows : — 



The grooves for the feet of the stand were cleaned, and the stand 

 placed with its feet resting in them. The trunnions of the micrometer 

 gauge were placed in the V-bearings ; the screw was then adjusted till 

 the lower end just touched the top of one rod ; by swinging the gauge, 

 which hangs by its trunnions in the bearings, this aclj ustment could be 

 done with great delicacy. 



The gauge was moved sideways by sliding the trunnions along the 

 bearings ; this horizontal movement brought the screw over the centre 

 of the second rod, and a second measurement was taken. This second 

 measurement, however, was not used. 



The tops of both rods were smooth, and a piece of copper was attached 

 to the iron rod in order to give a surface which would not corrode, 

 The micrometer screw was graduated to O01 mm., but as we had not 

 realised the importance of making sure that there was not a small 

 lateral displacement of the trunnions along the bearings, the last place 



