92 REPORT—1840. 
Another series gives— 
Mean oscillation at Zurich . «. . . « « O0°286 
os BS HO PRs a eS 
The two results agree very closely in assigning a value of the 
oscillation depending on the absolute pressure. M. Kamtz 
thinks that the diminution with height is as the diminished 
pressure, and may be expressed by 335th of the change of 
pressure. Thus, at the equator, the oscillation (according to 
him) would cease at a height where the barometer falls to 115 
lines, and afterwards, no doubt, would become negative, as I 
have formerly shown. ‘The observations cited by Col. Sykes* 
confirm the general principle. 
155. The cause of the diminution of the diurnal tide with 
height is no doubt this :—that the great vertical depth of air 
which exists between Geneva and the Great St. Bernard, for 
example, becoming heated by the action of the sun commencing 
at the earth’s surface, a portion of air is raised above the upper 
station in the afternoon, which in the morning was below it ; 
consequently this produces a diurnal tide in the higher regions, 
which has its maximum after the hottest part of the day, and 
which therefore counteracts the true diurnal tide. 
156. On the subject of the lunar influence on the barometer, 
we may refer, in the first place, to the popular article by M. 
Arago, in the Annuaire for 1833+; who gives the results of 
Flaugergues, mentioned in our last report{, which give a de- 
cided maximum at the last quarter, with which the observations 
of MM. Boussingault and Rivero, at Santa Fé de Bogota, 
agree. According to a late and complete reduction of the 
Paris observations by M. Eugene Bouvard§, we have a first 
maximum on the 8th day of the moon, and a second or prin- 
cipal one on the 22nd day; the principal minimum on the 
13th, and a second on the 27th day. Here, therefore, we have, 
as above, a decided maximum about the last quarter. The 
oscillation is 1*°78mm. Mr. Snow Harris has arrived at the 
same result as respects the principal maximum||, which may 
probably be considered as established. We shall return to this 
subject in treating of the fall of rain, and dependence of weather 
on the lunar phases. 
D. Barometric Variation with Height]. 
157. I will adda few observations on this subject, especially 
* Phil. Trans. 1835, p. 176. + P. 178. t P. 234. 
§ Correspondance de l’ Observatoire de Bruxelles, tom. viii. 
|| Athenzum, Sept. 14th, 1839. 
q Last Report, p. 236. Mahlmann, p. 119. 
$$ eee——es—SsSsS— = 
