A region of calms between 30 and 35 North; so called because ships laden with horses bound to America or the West Indies were often obliged to lighten their freight by casting the horses overboard when calmbound in these latitudes.
“Nothing could have been more delightful than our run into the horse
latitudes. Gales and dead calms, terrible thunderstorms and breezes,
fair one hour and foul the next, are the characteristics of these
parallels. Numbers of horses were exported from the mother country, and
it was reckoned that more of the animals died in these ... latitudes
than in all the rest of the passage.” —
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894