590 MR ALEX. BUCHAN ON THE MEAN PRESSURE OF THE ATMOSPHERE 



taken reversely, together with the isothermal lines taken directly. In other words, 

 the regions of lowest pressure, by giving the ascending currents, point out the sources 

 or fountains whence the upper currents flow ; and the isothermals, by showing 

 where, on account of the low temperature, the greater portion of the air is con- 

 densed in the lower beds, and so diminishing the pressure in the upper beds, point 

 out the regions towards and over which the upper currents diffuse themselves. 



1. To travellers in the interior of continents and regions at a great distance 

 from places where Meteorological Observations are made, the Charts will be of use 

 in showing the approximate sea-level pressures for each month of the year. The} r 

 show at a glance the zero points from which the heights of places may be calcu- 

 lated, at which observations of the pressure of the atmosphere have been made* 



2. To sailors, the Charts will be useful as showing the prevailing winds at 

 many places in each of the twelve months, and still further as suggesting, from 

 the connection which is here pointed out between mean atmospheric pressure and 

 prevailing winds, the winds which are most likely to be met with in regions 

 where little is known of the general course of the winds from actual observations. 

 The charts of least prevailing winds in January and July (Plate XXVII. ), will also 

 be useful in this respect. 



The following illustration will show the method of using the Charts in apply- 

 ing Buys Ballot's Law of the Winds. This law has been stated at the foot of 

 page 581, but it may be more popularly expressed thus, — Stand with your back 

 to the wind, and the low barometer will be to your left in the northern hemi- 

 sphere;! or > reversing it, stand with the high barometer to your right, and the low 

 barometer to your left, and the wind will blow on your back. Suppose, during 

 the summer months, a person at Lisbon to stand so, with reference to the high 

 pressure in the Atlantic, and the low pressure in Africa, he should have a N.N E. 

 wind ; and as he proceeded southward along the coast of Africa, the wind would 

 wear more to eastward. On the north coast of South America, being between the 

 high pressure of the Atlantic and the low pressure of South America, the winds 

 should be about easterly ; and on the north coast of Central America, the low 

 pressure in the Pacific being now to his left, the winds should be about N.N.E. 

 On passing through the West Indies towards Florida and the south-eastern States, 

 as the influence of the low pressure in North America in its relations to the high 

 pressure in the Atlantic comes into play, the prevailing winds should gradually 

 become E., E.S.E., S., S.S.W., and S.W., and from this region to England to about 

 W.S.W. These are, it need scarcely be said, the prevailing winds of these regions. 



* Note of the Determination of Heights, chiefly in the interior of Continents, from Observations 

 •M" Atmospheric Pressure. — "Proceedings of the Society," vol. vi. p. 465. 

 f In the southern hemisphere the low barometer will be to the right. 



