SCIENTIFIC SUMMARY. 
109 
PHYSICS. 
A Comparison of the Different Methods of Aerial Navigation. — This is 
made in a recent paper (published in the u Comptes Bendus ” for Nov. 9, 
1874), by M. Duroy de Bruignac. He divides all methods into two sys- 
tems, the Aerostat and the Aeroplane ; and he also alludes to a mixed 
system, the Mixed Aeroplane. The paper is partly mathematical, and there- 
fore unsuited to most of our readers, but we refer to it as some of them 
may be interested in knowing of it. The subject appears to be fairly 
worked out. 
A Patent Recording Thermometer has been lately devised by Messrs. 
Negretti and Zambia, which bids fair to supersede all former plans of instru- 
ment. It is said of it by the makers that it contains only mercury, without 
any admixture of alcohol or other fluid. It has no indices or springs, and 
its indications are by the column of mercury. It can be carried in any posi- 
tion, and cannot possibly be put out of order, except by actual breakage of 
the instrument. 
The Earliest Scientific Notice of Mirage. — In a paper by Professor 
Everett, published in the “ Proceedings of the Belfast Natural History 
Society ” [1874], he says that the earliest explanation of mirage on record 
is that of Monge [Ann. de Chim. xxix., 207], one of the savans who accom- 
panied Bonaparte in his expedition to Egypt. The following is the passage 
in the Annals, which purports to be an abstract of a memoir read at a meet- 
ing of the Institute, held at Cairo : u At sea it often happens that a ship 
seen from afar appears to be floating in the sky, and not to be supported by 
the water. An analogous effect was witnessed by all the French during the 
march of the army across the desert. The villages seen in the distance 
appeared to be built upon an island in the midst of a lake. As the observer 
approached them, the boundary of the apparent water retreated, and on 
nearing the village it disappeared, to recommence for the next village. 
Citizen Monge attributes this effect to the diminution of density of the 
inferior layer of the atmosphere. This diminution in the desert is produced 
by the augmentation of temperature, which is the result of the heat com- 
municated by the sun to the sands with which this layer is in immediate 
contact. ... In this state of things the rays of light which come from the 
lower parts of the sky, having arrived at the surface which separates the 
less denser layer from those which are above, do not penetrate this layer ; 
they are reflected, and thus form in the eye of the observer an image of the 
sky. He thus sees what looks like a portion of the sky beneath the horizon, 
and it is this which he takes for water.” 
The Electric System of Gas Lighting at the National Assembly at Ver- 
sailles. — We do not know whether the method adopted is the same as that 
which is employed in lighting the three thousand seven hundred jets at the 
Albert Hall in South Kensington, but according to M. Lissajous [in the 
“ Bull.de la Soc. d’Encouragementpour l’lndustrie Nationale,” Nov. 10, 1874], 
it is as follows : A Ruhmkorff coil of medium size, with an automatic mer- 
curial interruptor, is set in action by a Leclanche battery of four elements, 
the zincs having a surface of 4 square decimetres. These are only equiva- 
