126 LECTURES TO SCIENCE TEACHERS. 



of the earth may be taken roughly at 3,950 miles, the 

 depth of one mile of sea-water will diminish the total 

 gravitation by 3 g \ part, or a fathom depth of sea-water will 

 dimmish the total gravitation by ^,y^J,ou ^ part. This calcu- 

 lation is of importance as showing that the decrease of 

 gravitation must be proportionate to the depth ; but this 

 result is of course influenced, as I have already mentioned, 

 by latitude and by other disturbing causes, which have to 

 be allowed for in dealing with these instruments, unless they 

 are corrected in the instrument itself, and that is a subject 

 which we shall have to consider. 



Besides the disturbance in the total gravitation due to 

 latitude, there is one which was first referred to by Sir 

 George Airy, and which Professor Stokes has assigned to 

 its true cause, namely, that the total attraction of the 

 earth is always greater on islands than on the mainland. 

 On the shore of the mainland the total attraction of the 

 earth in the same latitude is less than on the shore 

 of an island, and Professor Stokes proves that this is 

 owing to the attraction of the mass of the continent 

 itself. Its mass lying above the general surface of the 

 earth exercises naturally a negative influence upon 

 the total gravitation, and it follows that the total 

 gravitation at the sea-level must be less in proximity to 

 large continents than it would be on the shore of a small 

 island. It has also been proved by Archdeacon Pratt and 

 others that the attraction of mainlands upon the water 

 itself must draw the water towards this mainland, and 

 that hence the water-level near shores must stand higher 

 than the water-level at a greater distance from the shore. 

 In other words, a continent draws the water towards it 

 and raises the sea- level near shore. I have received a paper 

 from Dr. Hann which deals with the same subject, and he 

 goes farther than those who dealt with it before him, and 

 maintains that the total attraction of the earth owing to 

 this continental local attraction would be greater at mid- 

 ocean than near shores that is to say, that the depression 

 of the water-level would be such that the gravitation on 

 the surface of the ocean would actually be greater than the 

 total attraction on the sea-shore. If Dr. Hann is right I 

 am wrong, because the instrument which I have the honour 

 of bringing before you is based upon an exactly opposite 



