84 PLYMOUTH INSTITUTION. 



phenomena of the tides. It was shewn that the tides result from 

 the unequal attraction of the sun and moon on the centre and sur- 

 face of our earth ; the waters being more attracted at the nearest 

 surface than the solid nucleus at the centre, and the waters on the 

 opposite side are less attracted than the central parts ; by these 

 unequal forces the waters are raised above their medium level, imme- 

 diately under the moon, and also on the opposite side of the earth ; 

 and depressed below their medium level, at parts 90 degrees on 

 each side ; the elevation corresponding to high water, the depression 

 to low water. 



The lecturer explained the cause of the unequal height of the 

 morning and evening tide, of the same day, as resulting from the 

 alternate sojourn of the sun in the northern and southern hemis- 

 pheres ; he shewed, by a diagram, that, in consequence of the rota- 

 tion of the earth on its axis, any place in north or south latitude 

 must in summer or winter pass alternately through a superior and 

 an inferior tide, because the poles of the tide are drawn by the sun 

 from the equator towards the tropics, and there will be alternately, 

 every twelve hours, a high and a low tide. 



It was shewn that the solar and lunar tides coexist, and that 

 their sum produces springs, and their difference neap tides ; that 

 there are two elevations and two depressions of the water's surface, 

 at 180 degrees from each other ; that the elevations have been named 

 " tide waves ;" that these elevations travel in different localities, 

 with different velocities, varying from 800 to 2 miles an hour: 

 that the greatest velocity is in the open and deep ocean, the least up 

 the meandering and shallow bed of a river: that the tide penetrates 

 600 miles up the River Amazon, and to a height of 300 feet above 

 the level of the sea. 



Many other singular and interesting tide phenomena were noticed 

 and explained, and the lecturer stated that the tide undulation 

 which fills our ports is derived from a tide wave, originating in that 

 zone of water which begirds our globe in latitudes south of Cape 

 Horn, and that it is at least two days in travelling to our shores. 

 He then proceeded to detail the results of between 2 and 3,000 tide 

 observations made in Plymouth Dock Yard, by means of a self-re- 

 gistering tide gauge, whereby some new and interesting laws have 

 been elicited, which, being made in our neighbourhood, may be 

 acceptable to our readers. It has been ascertained that every tide 

 ranges as much above as it falls below a certain point; that this 

 point may be considered the medium level of the sea, when undis- 

 turbed by winds, waves, or tides; it should be used as a standard 



