Dana is describing progress on homeward voyage, and mentions the "Horse Latitudes". Horrible.
horse latitudes
5said to be so named because sailing vessels transporting horses to the West Indies often had to throw horses overboard because of water shortages resulting from delays while becalmed6 either of two belts over the oceans at c. 30U 3 35U north and south latitude, characterized by calms, light wind, high barometric pressure, and hot, dry weather

Omigosh, wwh! I've heard of horse latitudes on and off over the years and never questioned it. What a sad tale, the one you've told us here. And, by coincidence, we heard today of a sad local tale--15 horses removed from some land close by because the horses were almost starved by their owner.