1815.] Royal Institute of France. © 459 
rism whose angle was parallel to the axis of the needle, which is 
likewise that of the primitive rhomboid. If we look at the flame of 
a candle through this prism, when we direct the eye through the 
thinnest part, we see two images the brilliancy of which is sensibly 
the same; one of which, the ordinary, is polarized in the direction 
of the axis of the tourmaline; and the second, extraordinary, in the 
direction perpendicular to that axis. But in proportion as we pass 
~to the thicker part of the prism, the ordinary image becomes 
weaker, and at last disappears entirely; while the extraordinary 
image continues to be transmitted without undergoing any other: 
diminution of density than what proceeds from absorption. 
This property occasions various phenomena, which are easily 
foreseen when known, and which experiment confirms. They have 
much analogy with those that Dr. Brewster has discovered-in the 
agate. In examining these, M. Biot has ascertained that they do 
not take place as in the tourmaline, but beyond certain limits of 
thickness ; for when the agate is made sufficiently thin, it possesses 
all the properties of crystals endowed with double refraction. ; 
The memoir of M. le Baron Ramond on the Meteorological 
Operations performed at. Clermont-Ferrand since the Month of 
June, 1806, to the end of 1813, offers a general table of the 
observations which the author has made with the best instruments; 
the most assiduous care, and the precautions which a long expe- 
rience had shown him to be necessary. They embrace the seven 
years of his residence in a department which offers so many objects 
interesting to the curiosity of the philosopher and naturalist. In 
a preceding memoir M. Ramond had shown the method of deter- 
mining by the barometer the relative heights of two stations at a 
distance from each other. Here he gives merely the annual and 
diurnal heights of the barometer and thermometer, the influence of 
seasons and the time of the day, and the periodical oscillations ob- 
served in the height of the mercury. 
The memoir is terminated by three tables, each of which offers 
the mean results of several thousand observations. In the first we 
find the mean height of the barometer for seven years, the mean 
heights of each season, those of each month at mid-day, with the 
- oscillations at three other epochs, the most critical of the day : 
finally, the accidental variations, their extremes, and the variationy 
for each season and each month, 
The second presents with the same detail the degree of the centi- 
grade thermometer at mid-day, with the extreme heights and the 
mean variations, 
‘The third is consecrated to the meteors. We see the direction 
of the wind, the number of rainy and snowy days, of hail, of blight, 
of fog, of hoar frost, of frost, of strong wind, of thunder and light- 
ning, of cloudy and cloudless days. 
Of these three tables the first is that which furnishes the greatest 
number of interesting remarks. We could not give an exact idea 
of it except by copying the whole memoir, which is itself only an 
