108 DESCRIPTIONS OF EGYPT. 
medium heat at Cairo is from 90° to 92°, and 
in winter from 58° to 60° ; the difference between 
the greatest degrees of heat and cold seldom ex- 
ceeding thirty degrees of Fahrenheit's thermometer. 
On some occasions, however, it has been known to 
rise to 112° ; but such an uncommon heat is ge- 
nerally of short continuance, and occurs most fre- 
quently in the Sahid. * At sunset the winds fall, 
the temperature of the air becomes cooler, and the 
superabundant humidity, which the heat exhaled, 
but could not elevate in the atmosphere, is again 
deposited in the form of dew. As the evening de- 
scends, a thin mist veils the horizon, and broods 
over the watery grounds, but in the darkness it be- 
comes scarcely perceptible, and in the morning, 
when the sun rises, quickly disperses in flaky clouds, 
Nor are the clouds always dispelled by the action 
of the sun's rays ; for the atmosphere sometimes 
appears loaded, and exhibits all the meteorological 
symptoms which indicate rain in other climates, 
* The mean heat at Cairo, during the different months, is 
thus given, according to Reaumur's thermometer, by Cotte, 
an accurate meteorologist: Number of observations in the day, 
three. January 11° 0'— February 10° 9'— March 14° 5' — 
April 16° 5'— May 20° 5'— -June 22° 7'— July 23° 7'— Au- 
gust 24° 2' — September 21° 6' — October 1 9° 4' — Novem- 
ber 17° 4' — December 12° 5' Mean heat of the year 
I70 9'.— Journal de Physique, July 17.91. 
