lOO 



A. A. MICHELSON 



to which it is subject in consequence of the attraction of the sun 

 and moon and as modified by the resulting distortion of the body 

 of the earth. But this may be furnished with any desired degree 

 of accuracy by the changes in the position of the level of a Hquid 

 surface which is necessarily normal to the resultant of all the forces 

 acting. If from these we can eHminate all but the gravitational 

 forces the problem is solved. 



Fig. I. — Microscope and gauge 



A very sensitive method of measuring the changes in level is 

 furnished by the interferometer; and a method of carrying this 

 into practice was devised and the apparatus constructed^ in 1910. 



But before attempting to utiHze so delicate an appHance it was 

 deemed advisable to make these preHminary experiments with 

 the microscope. Accordingly a 6-inch pipe, 500 feet long, was 

 half filled with water,^ the level of which could be read off through 

 the glass sides of the end vessels, as shown in Fig. i. 



I An interference apparatus for this purpose was independently devised by Pro- 

 fessor A. G. Webster. 



^ The vessels at the ends were at first connected by a pipe filled mth water, but 

 with this arrangement temperature changes produced enormous disturbances in the 

 level. 



