ELECTRIC CRLUMN. 
Sll 
*'• time of the evening dew and sun rise : it then increased by 
** degrees, arrives, sooner or later, but always before the 
middle of the day, to a certain maximum, whence it seems 
" gradually to decrease, till the dew falls again.” 
After having related these observations of M. de Saussure, I author’* 
compared them, in the same work, with my observations on the obsenrations 
' 00 the atioos- 
changes of heat in the atmosphere during the same periods of pheric change* 
the day, in clear weather. I was led to these observations in 
my attempt to measure the heights by the barometer, which I 
have related in my work, Recherches sur les modifications de 
I’Atmosphere. There I first proved, that the unsuccessful 
attempts of former experimental philosophers, which had 
finally produced among them the opinion, that this measure* 
ment could not be submitted to any rule, on account of un* 
known changes in the nature of the air, was principally owing 
to their not having taken notice of a cause on which greatly 
depends the degree of density of the air, viz. the difference of 
heat ; and that the attempt to make this measurement would be 
always unsuccessful, if the actual temperature of the air was 
not introduced as a datum in the formula. 
When I had discovered this necessary condition, it imme* 
diately reconciled together many of my observations which gave 
very different results at the same height on a mountain ; because 
I could judge, from other circumstances, that the temperature 
was different. Therefore, with the view of accurately determin- 
ing the effect of the differences of heat on the pressure of that 
column of air which produced the difference of height in the 
barometer at the foot, compared with that in certain parts of the 
mountain, I often observed the correspondent changes of heat 
•nd of differences of height in the barometer, from sun rise 
to sun set. Thus I found generally, in clear weather, that the 
heat increases in the atmosphere from sun rise to about 1 o’clock 
in the afternoon, and then diminishes gradually till sun-set. 
This coincidence, with respect to periods, of the progress of The changes 
heat in the atmosphere, and that of atmospheric electricity 
observed by M. De Saussure, appears to me of great impor- have an agree- 
tance in meteorology. Thence we learn, that the sun’s rays ™e^riods! 
form 
