Sentence:  At the equator, an immense mist seems permanently to encircle the globe. It is known as the cloud-ring. The function of the cloud-ring is to temper the heat of the tropics. . . . These are what are called <WSD>horse latitudes</WSD>. It was here that navigators of bygone ages were accustomed to cast their horses into the sea to lighten the ship in stormy weather. 
Meaning of the word marked with <WSD> tags:  The warm, subtropical bands which encircle the globe between approximately 30 and 35 degrees both north and south of the Equator, characterized by high atmospheric pressure and dry variable winds ranging from calm to light.