LIBRATION • DICTIONARY ONE.COM • Definition of Libration

... north and south, whose uniform and invariable action causes a long and constant swell, that exists even in the calmest weather, about the line, towards which its direction tends from either side. This swell or libration of the sea is so prodigiously long, and the sensible effect of its height, of course, so much diminished, that it is not often attended to; the gradual slope engrossing almost the whole horizon when the eye is not very much elevated above its surface: ...

— The History of Sumatra - Containing An Account Of The Government, Laws, Customs And - Manners Of The Native Inhabitants • William Marsden