562 ADDITIONS. 



But the statical effect of the attraction of manageable masses, 01 

 even of mountains, is very small. The effect of a small change in 

 gravity may be accumulated by being constantly repeated in the oscil- 

 lations of a pendulum, and thus may become perceptible. Mr. Airy 

 attempted to determine the density of the Earth by a method depend- 

 ing on this view. A pendulum oscillating at the surface was to be 

 compared with an equal pendulum at a great depth below the surface. 

 The difference of their rates would disclose the different force of grav- 

 ity at the two positions ; and hence, the density of the Earth. In 

 1826 and 1828, Mr. Airy attempted this experiment at the copper 

 mine of Dolcoath in Cornwall, but failed from various causes. But in 

 1854, he resumed it at the Harton coal mine in Durham, the depth of 

 which is 1260 feet; having in this new trial, the advantage of trans- 

 mitting the time from one station to the other by the instantaneous 

 effect of galvanism, instead of by portable batches. The result was a 

 density of 6 - 56 ; which is much larger than the preceding results, but, 

 as Mr. Airy holds, is entitled to compete with the others on at least 

 equal terms. 



Tides. 



I should be wanting in the expression of gratitude to those who 

 have practically assisted me in Researches on the Tides, if I did not 

 mention the grand series of Tide Observations made on the coast of 

 Europe and America in June, 1835, through the authority of the Board 

 of Admiralty, and the interposition of the late Duke of Wellington, at 

 that time Foreign Secretary. Tide observations were made for a 

 fortnight at all the Coast-guard stations of Great Britain and Ireland 

 in June, 1834; and these were repeated in June, 1835, with corre- 

 sponding observations on all the coasts of Europe, from the North 

 Cape of Norway to the Straits of Gibraltar; and from the mouth 

 of the St. Lawrence to the mouth of the Mississippi. The results of 

 these observations, which were very complete so far as the coast tides 

 were concerned, were given in the Philosophical Transactions for 

 1836. 



Additional accuracy respecting the Tides of the North American 

 ^oast may be expected from the survey now going on under the direc- 

 tion of Superintendent A. Bache. The Tides of the English Channel 

 have been further investigated, and the phenomena presented under 

 a new point of view by Admiral Beechey. 



