HORAEY VARIATIONS OF THE BAROMETER 415 



struck nur tentf, which were pitched in the plain of Al)d el Nour, b(^tweiMi Con 

 gtantine and SiJtif, th^■ outsidi- of these tents covered -with crystals of ice, and 

 the thermom'-ter laid ujton th" canvas has descended by at least half i degree 

 below uero. Th- sky was of the pm-est blue and the air uf wonderful trans 

 parency Tht; sun arose in full radiance, and before -'ight o'clock the heal had 

 becom'- as oppressive as it was the evening previous on our arrival at th^^ bivouac 

 If I remember rightly the thermometer t^tood thus early at 27° centigrade, and 

 it scarc^-ly reached a higher point during the whole day. 



From nine till ten o'clock of the morning, there is, as we have said, an equiU 

 briura between the force which tends to communicate an ascensional movement to 

 the column of air, and the resistance which that column opposes to the movem-nt 



From ten o'clock onward, the ascensional force decidedly prevails ; the inertia 

 has been overcome, motion from below upward- has been imparted to the whole 

 column, and there results, as it were, a suction or partial vacuum over the sur- 

 face of the earth, and consequently on the cistern of the barometer. The mercury 

 sinks in the tube, not because there is less air above this cistern, but because 

 that air ascending vertically has lost a portion of its gravity and of its elasticity 



The fall of the mercury, which commenced about ten o'clock, continues till three 

 at which hour, as observation proves, it is completed, and the barometer now in 

 dicates a minimum. There is nothing in this which does not correspond with 

 what might easily have been foreseen. The heat begins or will soon bf-gin to 

 diminish ; the solar wind or trade wind is about to lose its velocity — a ^ elocity 

 which in itself, also, has the eflFect of diminishing the pressure of the air m lln- 

 barometer. Everything concurs therefore to produce the descending movemeuf 

 of the column of air, a movement which, it may be said, is rendered visibl<- to 

 all who observe what occurs with regard to smoke or the corpuscles suspended 

 in the air, as soon as the sun begins to be inclined towards the horizon instead 

 of mounting, as they do in the middle of the day, they remain on the surface ui' 

 the ground. The perfume of flowers, instead of being dissipated in the atmo 

 sphere, is condensed and becomes stronger and more penetrating Everythiug 

 in effect, manifests a movement of the air towards th'- earth , th^- clouds sink 

 lower, the mists occupy the valleys, &c. 



The sm-face of the earth becoming cooler towards four or five o'clock, a downward 

 movement of the whole corresponding atmospheric column ensues; the air ac 

 quires a certain velocity, and this active force is represented to the sight Dy an 

 ascension of the mercury in the barometer. From observation the metal is found 

 to attain a new maximum of height about nine o'clock in the evening, twelve hours 

 after tlie former maximum in the morning. But, as has been already said, there 

 is some uncertainty aljout the precise instant of this second maximum, an un- 

 certainty which may be explained by the slowness with which the temperature 

 sinks during the night. The subsidence of the thermometer, from th'^ time when 

 the sun begins to descend towards the horizon to that of its setting, and even a 

 little after, is quite sensible, but afterwards, although the earth's surface continue.s 

 in reality to grow cooler, it is constantly brought into contact with successive 

 volumes of the descending air which has not yet lost all the heat acquired dur 

 ing the day, and through the effect of this contact there is established, not indeed 

 a complete equilibrium in the temperature, but, at least, a partial compensation 

 for the effect of the radiation of the surface dunng the night 



Th" ascension of the mercury in the barometer, from four or five o'(-lock in ihe 

 evening to about nine, would be certainly more rapid and. extended if the con 

 dcnsation of the air in the strata next to the earth did not take place simultane- 

 ously with another very common phenomenon, which in hot countries acquires 

 considerabb- proportions ; 1 speak now of the evening vapors and the dew. The 

 former begin to manifest themselves in intertropical regions as early as the 

 fourth hour of the afternoon, and observant travellers even report that there are 

 days on the Senegal when the phenomenon becoinL's perceptible witkin two 



