On an Experiment to determine the Earth* s density. 95 



3. Note regarding an Experiment suggested by Professor 

 Robison. 



In his memoir of Dr. Chalmers, lately read to this Society, 

 Mr. Ramsay has referred to an experiment which Dr. Chal- 

 mers was anxious to have performed on the tide-wave in the 

 Bay of Fundy. The object was to determine the earth's den- 

 sity by the attraction of the tide-wave on a plummet or spirit- 

 level, on the same principle as Maskelyne's experiment on 

 Schiehallion, but with the superior advantages arising from 

 the perfect homogeneity of the attracting mass, and from the 

 circumstance that all the observations might be made at a 

 single station. The experiment might, in short, appear to 

 unite the advantages both of Maskelyne's and Cavendish's 

 methods of determining the earth's density. 



The suggestion was Dr. Robison's, and Dr. Chalmers had 

 it from him. It is contained in the Elements of Mechanical 

 Philosophy, edit. 1804, page 339, and is given in the follow- 

 ing words : — " Perhaps a very sensible effect might be ob- 

 served at Annapolis- Royal in Nova Scotia, from the vast ad- 

 dition of matter brought on the coast twice every day by the 

 tides. The water rises there above 100 feet at spring-tide. 

 If a leaden pipe a few hundred feet long were laid on the level 

 beach at right angles with the coast, and a glass pipe set up- 

 right at each end, and the whole filled with water, the water 

 will rise at the outer end, and sink at the end next the land 

 as the tide rises. Such an alternate change of level would 

 give the most satisfactory evidence. Perhaps the effect might 

 be sensible on a very long plummet, or even a nice spirit- 

 level." 



It is needless to observe that the methods proposed by 

 Dr. Robison are not the best which might be suggested ; but 

 that, in consequence of the extreme simplicity of the observa- 

 tion, considered as a purely astronomical one, a deviation of 

 the direction of gravity of only a very few seconds could be 

 ascertained within small limits of error*. 



I thought it worth while to make the calculation approxi- 

 mately for an assumed height of the tide-wave. Had the 

 result been at all encouraging, I should have taken pains to 

 ascertain, on good authority, the exact rise of the tide, and 

 the circumstances of the locality whence the rise is greatest. 



* The micrometric observation of a plumb-line, as in a zenith sector, 

 would be sufficient ; or, as Professor Smyth has suggested to me, the view 

 of the wires of a transit-instrument, with a collimating eye-piece, as re- 

 flected in a mercury trough, — an observation, the accuracy of which may, 

 he states, be brought within one-twentieth of a second. 



