414 HOEAEY VARIATIONS OF THE BAROMETER 



as it were, a solstice, or, in nautical phrase, a period of slackwater. After ten, the 

 mercury begins to sink ; at noon it is at a height which is very nearly the mean 

 between the elevations of the day. From noon the column continues to sink, 

 and reaches the minimum of height about three o'clock. From that hour till 

 four of the afternoon we find a new interval of repose, the precursor of a change 

 of sign in the movement of the mercury ; and, finally, towards four or five 

 o'clock, the column begins again to ascend. 



About nine o'clock in the evening there is a second maximum in the height 

 of the mercury, but the precise moment of this maximum is not yet well known, 

 It would even seem that after an ascension, quite distinct and quite rapid, which 

 takes place from five to nine or ten o'clock of the evening, the mercury does 

 not absolutely cease to ascend, but that its ascensional movement, extremely 

 slow and difficult to appreciate justly, still continues for an hour or two, or 

 perhaps even three hours ; after which succeeds a stationary interval of some 

 length, and then a movement of subsidence, likewise very slow, and which may 

 even continue during the first hour which follows the rising of the sun. 



Such, in its principal circumstances, is the meteorological phenomenon to 

 which has been given the name of diurnal period. Let us proceed to the expla- 

 nation. 



Let us suppose oin-selves in the midst of a vast plain situated under the 

 tropics. It is five o'clock in the morning, the air calm, the weather clear, the 

 sun about to rise. It rises in effect, and its rays, at first tangents to the surface 

 of the earth, glide over without warming it, and without communicating, so to 

 speak, any heat to^ the air which is in contact with the ground. Presently, how- 

 ever, the luminous and calorific rays become inclined to the horizon, raise the 

 temperature of the points upon which they strike, and consequently also the 

 temperature of the air which touches the points warmed by the sun. These 

 inferior strata of the atmosphere, from mere change of temperature, tend to 

 dilate, to rise, and would in effect rise rapidly, were it not that in this ascen- 

 sional progress they have to overcome the resistance which the superior strata 

 oppose to their upward movement. It is necessary, therefore, that the air 

 already heated should overcome the inertia of that which as yet is not heated — 

 should displace and set it in motion. As long as the entire column, so to say, 

 has not participated in this movement from below upward, the pressure is in- 

 creased on the surface of the ground where the observer is supposed to be ; it 

 consequently increases also in the air which touches that surface, and hence 

 upon the cistern of the barometer, which here fulfils not only the function of 

 an instrument measuring the weight of the air, but also that of an instrument 

 measuring the elasticity or resistance of a volume of air comprehended within 

 certain limits of which the temperature continues to increase ; in a word, our 

 instrument is as well a manometer as barometer. And, let it be said in passing, 

 it is because this double ofiice of the instrument; which we owe to the genius of 

 Pascal and Torricelli, has been too long overlooked that we have remained so 

 much behindhand as regards the explanation of a great part of the phenomena 

 of meteorology. But we return to the diurnal period. 



In the struggle of which we have just now spoken, and which takes place 

 between the atmospheric column and the part of the air which is in contact with 

 the ground heated by the sun, now ascending more and more above the horizon, 

 there is an instant when the resistance is a maximum, and when, consequently, 

 the column of mercury attains its greatest height. Observation shows that this 

 maximum occurs about nine o'clock in the morning. 



There is nothing that need surprise us in this result of observation. On one 

 hand, in warm countries, the greatest heat of the day prevails from ten o'clock 

 in the morning or thereabouts ; on the other hand, it is from eight to nine o'clock 

 that the ascensional movement of the thermometer is most rapid. We have 

 seen in Algeria, at the commencement of September, at the moment when we 



