634 WOWDEHS OF AET. 
tion but tbeir enlarged field, and their freedom from pri*' 
matic colours iii that field j points of no consequence 
looking through a fixed glass at a fixed and circumscribs® 
object. The field of the Galilean telescope is quite lar^® 
enough, and having, instead of the six contained in Dollond^ 
achroinatics, but two lenses, one of which is a thin coU' 
cave, it exhibits the object with greater brightness, an“ 
therefore ought to have been preferred for this purpose. 
seems strange also, that, to ease the operator, it has nevet 
been contrived to exhibit the fixed spectrum on the principle 
of a portable camera, so that, without wearying the eye, 
changes of the distant telegraph might have been exhibit^" 
on a plane surface, and seen with both eyes like tlie leaf of* 
book. 
THE AIR BALLOON. 
Among the many discoveries of modem philosophy this 
one of the most splendid : hitherto, however, it has not bee* 
attended by corresponding utility, owing to the difficulty 
steering the machine. The most promising attempts ** 
overcome this difficulty were made by Zarabeccari, an Italic* 
whose aerial excursions are curiously detailed by Kotzebu®’ 
in his travels, and whose principles were truly scientific' 
but still this great desideratum remains to be attained. 
The discovery of hydrogen gas, which is 15 times ligh**^ 
than atmospheric air, suggested die plan of filling with tb** 
gaseous substance a silken balloon, and of its ascent in d*' 
with an aeronaut appended to it, provided tlie whole show 
not exceed the weight of an equal bulk of atmosphC'*; 
air. The process of filling the balloon is accomplished b,^ 
mixing five parts of water with one of sulphuric add, ** _ 
];ouring the mixture on iron filings ; the light gas, by '‘'‘j 
decomposition of the water, will rise into the balloon ; * 1 '" 
ll;e balloon, being 12 times lighter than die atmospheric a*’ 
win rise through it. Thus have two, three, and eV^ 
four persons, been at one time carried through 
atmosphere. 
More than fifty aerial voyages, in different parts of Eui'cp' 
have been made by Blanchard 5 nearly as many by G-u nO* ' 
and tlilrty by Mr. Sadler, 
