1876.] Valuations of the Barometer in India. 25 



sufficient, we fail to find it. From my own experience within the 

 tropics, where the great diurnal oscillations of the barometer have to be 

 explained by these currents, we may look in vain for any traces of them 

 during weeks of the season when the oscillations are most marked. At 

 stations near the sea, a slight breeze from the ocean during the day and 

 a like breeze seaward during the night are the only movements perceived 

 for days together ; while at inland stations even these are unfelt, though 

 occasionally local gusts of air sweep in one direction or another. From 

 the summits of the South-Indian Grhats, where the Coromandel sea 

 borders the eastern and the Malabar Sea the western horizon, clouds 

 could be seen forming over sea and land, which scarcely moved, and dis- 

 appeared near the places of their birth ; the highest cirri which mottled 

 the sky seemed frequently a fixed fretwork, or one which moved so slowly 

 that the forms had changed before the direction of their motion could 

 be determined. The clouds which occasionally ascended from the A'al- 

 leys to the mountain- tops remained so balanced in the air that the 

 size of their droplets could be estimated under the object-glass of a micro- 

 scope. During these days of calm from sea to sea, the barometer rises 

 and falls, on the highest peaks as on the plains, ^vith the regularity 

 of clockwork. 



When we transfer ourselves to the higher latitudes of the British 

 Islands and seek for evidence of these currents, no such regular move- 

 ments as the hypothesis requires can be observed : the only appearance 

 of the current from the pole is to be found in occasional north-east sur- 

 face-winds ; the upper currents, as shown by the motion of the cirri, 

 proceed on the average from the \s est. Here also the currents fail to 

 follow their supposed courses. 



The vapour-pressure supplementary theory fails completely whenever 

 the diurnal variation of vapour-pressure is small compared with that of 

 the barometer ; its apparent success in other cases is a mere arithmetical 

 result which could never support a careful comparison with facts. 



When we turn our attention to the changes of barometric height 

 \^'hich occur from day to day, the h^-pothesis of aerial currents seems 

 to have a surer basis. These variations have been carefully studied by 

 Sir J. Herschel, Mr. Birt, A. Quetelet, and others. The first considered 

 that these oscillations " perhaps take their rise in local and temporary 

 causes prevailing over great areas simultaneously, the principal no doubt 

 depending on the prevalence of cloud, or clear sky, or dryness, over 

 great tracts for several days or weeks in succession "*. He also thought 

 that the movements of the atmosphere thus produced should be a cause 

 of winds alternately progressive and retrogressive. 



Quetelet, who projected the lines of maximum and minimum pressure 

 on the map of Europe, suggested that the tropical current descending 

 * Eeport Brit. Assoc. lSn3. p. 99. 



