* 
390 Correspondence of J. Nickles. 
and greased. These two pistons are united together by an iron 
cord or wire when used with a musket, or by an iron chain 
from a meter to a hundred meters in length when with cannon. 
The pistons serve as projectiles; when fired, they straiten the 
chain between them, and flying through the air, they sweep every 
thing before them. 
Photography—Heliozraphic engraving.—The following pro- 
cess invented by M. Baldus, appears to bring to perfection the . 
method of engraving by the aid of the sun. The results ob- 
tained are very beautiful; and although the author has not de- 
scribed to us fully all the details, we know enough to give a gen- 
eral idea of his method. : 
On a plate of copper covered with petroleum a photographic 
proof on paper of the object to be engraved is placed; this proo 
is a positive, and will necessarily make a negative on the metal 
by the action of the light. After an exposure of a quarter of an 
hour to the sun, the image is reproduced on the resinous coating, 
but it is not yet visible; it is made to appear by washing the 
plate with a solvent which removes the parts not impressed b 
the light, and brings out a negative picture made by the resinous 
tracings of the bitumen. he designs are very delicate ; the tra- 
cings receive solidity by an exposure during two days to the ac- 
tion of a diffuse light. When thus hardened, the plate of met 
is plunged into a bath of sulphate of copper and is then connect- 
ed with the pole of a battery; if with the negative pole, a layer 
of copper in relief is deposited on the parts of the metal not pro- 
tected by the resinous coating; if with the positive pole, the me- 
tal is graved out in the same parts, and thus an etched engraving 
is obtained. 
So that at will a raised or etched engraving may be made, the 
former to be printed like a wood-cut, the latter like ordinary COP 
per plate engraving. 
Collodion.—At one of the recent sessions of the Academy of 
Sciences, MM. Bisson brothers exhibited a photograph © 
principal front of the Louvre; it was a positive on paper, 140 
' centimeters in length and 60 high, produced from a negative 
on collodionised glass. It consisted of 3 separate photographs, aS 
similar in tone of coloring as if taken at a single operation. Th 
operation was made with “collodion anticipé,” the plates having 
been prepared in the workshop, and carried to the place after 
having been rendered sensitive; the authors affirm that these 
plates preserve their sensitiveness;for several hours. 
Société d’ Encouragement pour 0 Industrie Nationale—We 
