Meteorological observations and essays / By John Dalton.

  • John Dalton
Date:
1834
Catalogue details

Licence: Public Domain Mark

Credit: Meteorological observations and essays / By John Dalton. Source: Wellcome Collection.

    236/280 (page 210)
    since then traced through the temperate zone, though it is there blended with other and more powerful variations, from other causes. Generally speaking, the fact is this, that early in the morning, about sun-rise, or soon after, the baro- meter is higher, all other circumstances the same, than it is afterwards ; that it droops a little, as the heat of the day advances, and is lowest nearly in the warmest part of the day ; after which, it rises as the air cools, and in the evening nearly recovers what it had lost since morning. The sun’s power being greatest in the torrid zone, this effect of it, (for it is evidently an effect of tem- perature,) is there a maximum ; and on this account it is more conspicuous there, as well as on account of the other variations being of less magnitude than in the temperate and frigid zones. The effect diminishes, in leaving the equator, in some propor- tion as the latitude, the seasons, and other circum- stances. The sun is constantly heating the earth and air, successively from east to west : the air being heated expands in various directions, to restore an equili~ brium of pressure ; if this expansion was only in a perpendicular direction, it would not disturb the barometer ; but as the air will go in any direction where the pressure is least, it has a lateral motion as well as a perpendicular one; and hence the column pressing on the mercury is less in quantity during the high temperature ; but when the excess of temperature is withdrawn, the air falls back into its former position.