22 



THE VOYAGE OF ELKS. CHALLENGES. 



situated to westward of a pretty extensive tract of land, the forenoon maximum is 

 retarded, — in some places as much as seven hours, — and that in all these cases the after- 

 noon minimum is small and in some instances all but disappears. On the other hand, 

 at no great distance from the coast, both inland and seaward, the afternoon minimum is 

 quite distinctly seen, the retardation of the forenoon maximum rapidly gives way, and 

 the chief phases of the diurnal oscillation occur near the normal times. This disturbance 

 in the diurnal oscillation can scarcely be said to occur on the east coasts of tracts of land. 

 This peculiarity of the diurnal barometric tide is due to the circumstance that the 

 air over the land is earlier and more rapidly heated than the air over the sea to west- 

 ward of it, and, consequently, the ascending current sets in sooner and stronger over 

 the land than over the sea, accompanied with the necessary result of the propagation of 

 a temporary overflow to westward by a sub-upper current from the continental toward 

 the insular situation. The retardation of the phases of the curve is also seen in lower 

 latitudes, though less easily detected owing to the larger amounts of the oscillations. 

 In summer, at Coimbra, pressure falls to the mean of the day shortly after -noon, but 

 at Lisbon it is an hour and at San Fernando an hour and a half later. At Milan it 

 occurs about 12.45 p.m., but at Naples it is delayed to about 3 p.m. 



The following Table presents another set of diurnal barometric curves totally different 

 from any yet referred to. It gives in thousandths of an inch, the winter, summer, and 

 annual means for Gries, Klagenfurt, and Cordova, to which Mexico is added. 



