1877.] (tnd Land- and Sea-lreezes. 4i7 



which, in the hot weather, sets in about the hour of the evening drive, is at 

 its height about the usual dinner-hour, and makes the south verandah so 

 pleasant for the post-prandial lounge. It lulls gradually, and as a rule 

 ceases to be felt about midnight. Meanwhile, the cooling of the lower and 

 more heated strata of the air, by reducing their elasticity, allows the 

 higher strata to sink under the influence of gravity ; and this, the more 

 rapidly, the faster the cooling proceeds ; and since, in virtue of the momentum 

 acquired during the interval of more rapid cooling, the motion continues, 

 after the contraction has begun to relax, the lower strata are dynamically 

 compressed, producing the evening maximum of barometric j)ressure. As 

 the expansion in the morning is greatest over the land, so also is the con- 

 traction in the evening ; and, owing to this, the isobaric planes at a certain 

 elevation are again disturbed, sinking lower over the land and producing a 

 head of j)i^essure over the sea. This disturbance causes a transfer of air 

 from over the sea to the land at the higher level ; and is followed by an 

 accumulation of pressure at the land-surface, causing the outflow of the 

 land-wind in the early morning hours. 



According to this theory, then, there should be an excess of pressure 

 over the land in the morning and as long as the land-wind prevails, and an 

 excess of pressure over the sea in the afternoon and evening ; and we might 

 expect that, at the time of the afternoon minimum, when, according to the 

 theory of the barometric tides, the air is exercising simply a static pressure, 

 the minimum of the sea- curve would shew a much less depression than the 

 land-curve^ while the reverse would hold good at the time of the morning mi- 

 nimum. A comparison of the Calcutta curve \vith that of the head of the 

 Bay, superimposed in the accompanying diagram, shews that these relations 

 really obtain in nature. Any small increase in the amplitude of the marine 

 barometric curve which may be required to render it strictly comparable with 

 that of the Calcutta standard barometer, would only have the effect of 

 increasing the difference of the night maximum and minimum, and 

 somewhat diminishing that of the morning maximum and afternoon 

 minimum. 



In the diagram, I have represented the curves as deviations from the 

 same line of mean pressure. Actually, in January, the mean pressure of the 

 land is in general lower than that over the sea. It must be remembered also 

 that the places represented are 80 or 90 miles from the coast-line ; and 

 therefore quite on the limits of the belt within which the daily oscillation 

 of the surface-winds is experienced. I anticipate that, when the data for 

 March and April shall be brought into comparison in like manner, the two 

 curves will shew a still greater difference, indicating a greater transfer of air. 



The following are the values for the six hours of observation, deduced 

 from the marine registers j the co-efficients of Bessel's formula, computed 



