274 Lieut. Maury on the Probable Relation between 



zone towards the pole ; in other words if s represent the total 

 volume of atmosphere which blows on the surface towards 

 the north on the polar side of Cancer, and the total 

 volume which moves on the surface from the pole towards 

 the calms of Cancer, then s being the rule, and the excep- 

 tion, we shall have s^ (p — s\ Therefore / is the quantity 

 which must return in the upper regions of the atmosphere 

 from the Arctic regions to the calm zone of Cancer ; and if 

 we take 8" as the quantity which comes from the equator in 

 the upper regions of the atmosphere to this same zone of 

 calms, we shall have the momentum of 8" equal to the mo- 

 mentum of s', as proved by nature in the fact that she has 

 established near each tropic, a zone, or belt of calms. 



The Cancer zone of calms in the Atlantic ocean is known 

 to American seamen as the " Horse Latitudes," from the 

 circumstance that the vessels formerly engaged in carrying 

 horses from New England to the West Indies, found it so 

 difficult to cross this zone : they would often be detained in 

 the calms for many days, during which time the large cargo 

 of horses would exhaust the stock of water, become frantic 

 with thirst, and to save a part, the rest would have to be 

 thrown overboard ; hence the name of " Horse Latitudes'* 

 to the calms near the tropics of Cancer, and which I have 

 called by the name of that sign. 



This is the place where the upper currents of air repre- 

 sented by s' and 8'' meet ; they balance each other, produce a 

 calm, and descend to re-appear as surface winds, one blowing 

 to the north and the other to the south from this calm belt. 



Now / could not bring the vapours here which form the 

 rains that are precipitated between this calm belt and the 

 polar regions, because s' had already performed the circuit 

 as a surface wind between this zone and those regions ; it 

 had been subjected to a temperature far below zero, and had 

 given out all the moisture that a dew point so very low could 

 extract from it ; and as it had returned in the upper regions 

 of the atmosphere where it encountered no fluid surface to 

 replenish it with moisture, it had no vapour on its arrival 

 from the north at the calms of Cancer, to make rains of. 



Hence if ^ returned to the north as a surface wind after 



