4 
NATURE 
[AZay 4, 1871 

with the increase of elevation above the clouds if they be 
present, and on this subject he gives the following 
laws :— 
“The azure colour of the sky, though resembling the 
blue of the first order when the sky is viewed from the 
earth’s surface, becomes an exceedingly deep Prussian 
blue as we ascend, and, when viewed from the height of 
six or seven miles, is a deep blue of the second or third 
order. 2. The maximum polarising angle of the atmo- 
sphere, 45°, is the same as that of air, and not of water, 
which is 53°. 3. At the greatest height to which I have 
ascended, namely, at the height of five, six, and seven 
miles, where the blue is the brightest, the air is almost 
deprived of moisture. Hence it follows that the exceed- 
ingly deep Prussian blue cannot be produced by vesicles 
of water, but must be caused by reflection from the air, 
whose polarising angle is 45°. The faint blue which the 
sky exhibits at the earth’s surface is therefore not the blue 
of the first order, but merely the blue of the second or 
third order rendered paler by the light reflected from the 
aqueous vapour in the lower regions of the atmosphere. 































































FIG. 2—DRAGGING 
To appreciate all the beauty of cloud scenery when the air 
is loaded with moisture, an aérial voyage must be made on 
an autumn morning before sunrise, when the atmosphere 
is charged with the vapours of night.” 
Clouds were frequently met with to a height of 20,0ooft, 
or nearly four miles, and heavy rain at almost as great an 
altitude ; and on one occasion, while descending, rain fell 
on the balloon at a height of three miles, and then for the 
next 5,000ft. lower, it passed through a beautiful snowy 
scene; there were no flakes in the air, the snow was 
entirely composed of spicule of ice, of cross spicule at 
angles of 60°, and of an innumerable number of snow 
crystals, small in size, but of distinct and well-known 
forms easily recognisable as they fell and remained.on the 
coat. The drawings show many a_ beautiful scene— 
sunrise from a balloon, moonlight effects, a lunar halo, the 
shadow of a balloon on the clouds, sometimes surrounded 
by an aureole, though, perhaps, none more remarkable 
than the mirage represented in our first illustration. 


Humorous incidents occur here and there; as when 
the whole apparatus is taken by the French peasantry for 
“le diable ” himself, or when the travellers approaching the 
earth are required by too zealous gensdarmes to show 
their passports! And the adventures are not without 
their serious attendant dangers. More than once the 
diminished pressure and the intense cold produced so 
great a numbness and tendency to sleep that it has re- 
quired the greatest presence of mind for all control over the 
balloon not to be lost—and for ever. Life and limb were 
also not unfrequently endangered by the too sudden 
descents, sometimes to escape the imminent peril of an 
involuntary dip into the sea. Fig. 2 depicts the manner 
in which the “ Swallow,” haying Tissandier and de Fon- 
vielle on board as passengers, was dragged along the 
ground by a furious gale, and both those eminent aéronauts 
were considerably hurt and in danger of losing their 
lives. 
There is not much contribution in the volume to the 
mechanics of aérostation, and that mostly from the French 

““ ENYREPRENANT ” 
V.G, 3—TNE VALVE OF THE BALLOON 
contributors. We have drawings of the weighing machine 
and pulleys of the great Captive balloon of Chelsea, and 
Fig. 3 represents the valve of the “ Entreprenant ” balloon 
from which M. de Fonvielle attempted to take photographs 
of an eclipse of the moon. The book is one which will 
doubtless find a large circle of readers, and will greatly 
increase the public interest in aérostatics. 


OUR BOOK SHELF 
A Text-Book of Elementary Chemistry, Theoretical and 
Inorganic. By George F. Barker, M.D. (New Hayen, 
Conn, : C. C, Chatfield and Co., 1870, pp. 336.) 
THIS little book is evidently the result of much labour on 
the part of the author, and cannot fail to be of much 
value to students of chemistry. In the preface a list of 
books is given of which the author has made free use ; 
consequently the peculiarities of the systems of many 
chemists are to be found in the book; but though it can- 
not be said that any school has been followed, yet all are 
more or less represented. The prevailing ideas are that 
