RIGIDITY OF THE EARTH 



lOI 



This pipe was laid in a trench six feet deep, terminating in two 

 pits, eight feet square and ten feet deep, walled with concrete, in a 

 soil of sandy clay on the grounds of the Yerkes Observatory at 

 Williams Bay, Wisconsin. The direction (E.-W.) of the pipe was 

 laid out by measuring from the meridian line and is probably cor- 

 rect to within one foot in five hundred. The pipe was then care- 

 fully leveled, and after verifying the continuity of the water and 



Fig. 2. — Point and totally reflected image 



of the air space above it, the pipe was closed so that it was 

 effectively air-tight, and the trench was filled with clay. 



The distance between the total reflection image of a pointer, 

 Fig. 2, and the direct image was read by micrometer microscopes 

 of about 2-inch focus. These were calibrated at first rather 

 roughly by measuring the diameter of a wire immersed in the water 

 at the focus; and subsequently with much greater accuracy by 

 measuring the known distance between two lines ruled on glass 

 and placed in the focus, under water. 



A preliminary series of observations was begun August 5 and 

 continued until September 2 with such encouraging results that it 



