928 



SCIENCE 



[N. S. Vol. XXXIX. No. 1017 



fractional part of the full computed value. 

 After this fraction has been found by experi- 

 ment, it is possible to compute the rigidity of 

 the earth. 



In 1879 George and Horace Darwin, at 

 Cambridge, England, attempted to measure 

 the rigidity of the earth. They used a heavy 

 pendulum whose motions were greatly magni- 

 fied by suspending a small mirror by two fibers 

 very close together, one of which was fastened 

 to the pendulum bob, and the other to a rigid 

 support. However, even with this extremely 

 sensitive apparatus they were unable to sepa- 

 rate any movements due to the moon's attrac- 

 tion from the multitude of disturbing dis- 

 placements caused by changes of temperature, 

 earth tremors, etc. 



Of these disturbing elements, a very serious 

 one is the distortion of the land caused by the 

 immense weight of water periodically thrown 

 upon the coasts in the ocean tides. As late 

 as 1898 Sir George Darwin said that he 

 doubted if it would ever be possible to isolate 

 the effects of the tidal forces from the 

 multitudinous disturbances of a more or less 

 accidental character, although he recognized 

 a possibility in the work of Paschwitz and his 

 successor Ehlert, at Strassburg. They had 

 already secured encouraging results by the use 

 of the horizontal pendulum, which is in effect 

 an exceedingly sensitive adaptation of the 

 plumb line. 



Since then measurements of this sort have 

 been carried out by von Eebeur, Kortazzi, 

 Hecker, Orloff and others. The experimental 

 results are so obscured by accidental disturb- 

 ances, however, that their interpretation is 

 difficult, and the results of these various ob- 

 servers differ widely. 



In recent years Professor T. C. Chamber- 

 lin has been much interested in the possible 

 effect of the earth tides on the ocean tides, 

 and he and Professor F. E. Moulton have for 

 years been anxious to secure some definite 

 data on the plasticity of the earth, on account 

 of its vital bearing on questions of planetary 

 evolution. It was through them that Professor 

 Michelson became interested in the problem of 

 measuring the elastic properties of the earth 



and designed the experiments which were con- 

 ducted last autumn on the grounds of the 

 Yerkes Observatory, at Williams Bay, Wis- 

 consin. These experiments are described in. 

 full, with graphs and tables of observations, 

 in the Journal of Geology and in the Astro- 

 physical Journal, for March, 1914. 



Professor Michelson's idea was to substitute 

 a long horizontal water surface for a long 

 pendulum, and measure the changes of level at 

 its ends. There are many advantages in this 

 arrangement. The length may be increased 

 to any desired extent. The water column may 

 be entirely under ground and thus, to a very 

 large extent, freed from temperature disturb- 

 ances. Earth tremors produce but little if 

 any effect. 



The arrangement actually used was as fol- 

 lows. An iron pipe, 500 feet long and 6 inches 

 in diameter, was buried in a trench 6 feet 

 deep along an accurately determined east and 

 west line. At each end there was a pit 10 feet 

 deep and 8 feet square, walled with concrete. 

 The pipe was leveled, certainly to within one 

 half inch, and probably to within one quarter 

 of an inch, and half filled with water. A 

 gauge at each end was connected at the top to 

 the air space in the pipe, and at the bottom to 

 the water. A needle point in each gauge was 

 brought up from below until it nearly touched 

 the surface of the water. The needle point 

 was illuminated through a window in one side 

 of the gauge by a small electric flash light. 

 The under surface of the water formed a very 

 perfect mirror, and the distance between the 

 pointer and its totally reflected image was 

 read, through a second window, with a microm- 

 eter microscope. Gauges, pipe and windows 

 were all air-tight, so that fluctuations in baro- 

 metric height at the ends were without effect. 

 If the gauge at one end was open, the image 

 at the other was not steady, but when both 

 ends were closed, the reflected image was as 

 steady and sharp as the pointer itself. Micro- 

 scopes were used for which a millimeter at 

 the focus corresponded to about 17 revolutions 

 of the micrometer head. Observations were 

 taken by measuring the distance between the 

 pointer and its image at one end of the pipe. 



