460 Proceedings of Philosophical Societies. [Dze. 
extract.drawn up with as much: clearness as conciseness... We 
shall select the remarks which are the most: interesting and the 
shortest. The mean results of the seven years differ very: little from 
what might, be deduced from the first two years. Hence they seem 
to possess jall the requisite certainty ;, and \it is not to be regretted 
that the author could not verify them by a longer abode: at) caclioy 
de Dome. 9 
The mean results of the seven | years interest more particularly the 
place that has furnished them. The variations observed in each 
season have a more general utility.. We observe in them:the-action 
of regular causes, which subject the atmosphere to periodical modi-+ 
fications. Each season has its character, In summer; the: mean 
height of the barometer is greater; in spring, it is less... Thespring 
is the epoch of the greatest diurnal oscillations. They are least:in 
winter. The accidental variations, on the contrary, are the greatest 
in winter, and the least in summer. 
We remark unequivocal annual oscillations in. the barometric 
mean, which seem analogous to the horary oscillations. ‘The mer- 
eury is highest in the month of January. It descends ‘till, the 
month of April, when it is lowest. It then mounts till the month 
of June; and after remaining elevated for some time, it descends 
till November, and mounts again rapidly to the height of January, 
The diurnal revolution has equally its annual phases. But pheno- 
mena so complicated would require a long continued series of good 
observations to determine what we cannot yet see in them. 
_ The hygrometrical observations do not appear in these tables. The 
author has ascertained that the variations in moisture have no sen- 
sible effect upon the state of the barometer. They were therefore 
indifferent to the main object which he had in view, and of conse- 
quence he did not examine them with the same assiduity. 
_ This new memoir offers a model which those persons »willno 
doubt follow who devote themselves to the study of the modifications 
of the atmosphere. It presents facts from which they may set out, 
either to give more exactness to the value of the mean pressures, or 
to employ these means more conveniently in barometrical measure- 
ments. We find here the complement of the different inquiries with 
which the author has occupied the Class at different times, and of 
which we have given extracts in our former reports. 
The memoir of M. Poisson on Elastic Surfaces is divided into 
two parts. The first is relative to fexible and non-elastic surfaces, 
of which M, Lagrange has given the equation of equilibrium inthe 
new edition of his Mecanique Analytique, i. 149. _M.. Poisson 
comes to the same equation by a different method, which has the 
advantage of showing the particular restriction under which itis, 
Tt supposes, in fact, a condition which is not often fulfilled, and 
which becomes impossible in a heavy surface of unequal thickness, 
To resolve the question completely, we must attend to the differ- 
ence of tensions which the same element experiences in two different 
directions. We then find equations of equilibrium, which include 
