1871.] Engineering. 139 



described at length some time ago in the " Chemical News." In selecting the 

 copper which is to enter into the composition of " standard gold," it is of course 

 needful that metal of considerable purity should be employed, and Mr. Roberts 

 proposes to test the copper by drawing it to fine wire, and then comparing its 

 electric conductivity with that of copper of known purity — the presence of a 

 small amount of foreign metals having a great effect upon this physical property 

 of copper. During the process of " blanching " or pickling the annealed blanks 

 for gold coin, the copper is dissolved from the surface and a thin film of pure 

 spongy gold produced, but during circulation this superficial film wears off, 

 and the standard thus becomes deteriorated. Mr. Roberts therefore proposes to 

 abolish the blanching of gold coin, and believes that this may be done without 

 affecting the beauty of the surface. In future the " sweep " of the Mint — by 

 which we understand the dust from the floors, ashes, and old crucibles — will 

 not be sold, but will be treated in the Mint, and it is proposed to extract the 

 precious metals from this refuse by the aid of Mr. Crookes's sodium-amalgam. 



ENGINEERING— CIVIL AND MECHANICAL. 

 Paris Defences. — The present war has, to a great extent, directed the atten- 

 tion of engineers to the subject of offensive and defensive works, in supercession 

 of those of progress and improvement. Naturally, all thoughts are directed to- 

 wards the beleaguered city of Paris, and speculation is rife as to her chances of 

 a successful resistance to the armed band by which she is encircled. The 

 water-gates of the city have been protected by movable dams and pontoons, 

 leaving just room enough for the passage of barges. The pontoons are com- 

 posed of iron plate, and pierced with loopholes for riflemen; they are moored 

 to anchors in the bed of the river, and similar defences are placed under the 

 arches of the bridges. A new kind of defensive armour has been invented for 

 the use of riflemen, called a pelle cuirasse, which consists of a light steel shovel, 

 with a handle of strong but light wood ; the tool is used for digging a pit 

 where the soil is suitable, and this done, the rifleman may stick the handle 

 in the ground and protect himself behind the shovel. Two or three great 

 locomotive turrets for mitrailleuses have been constructed for the defence, and rails 

 have been laid down within the ramparts for moving them wherever they may 

 be required ; and MM. Cail and Co. are now employed in the construction of 

 an immense armour-plated locomotive to carry three guns, as well as in the 

 manufacture of Chassepot barrels out of steel rails and axles which were in 

 store at the depot of the Western Railway. The iron gun-boats on the Seine 

 each carry a piece of 24 cwts. in the stern, and two mitrailleuses forward, and 

 are pierced in the sides with fifty loopholes for fusiliers on each side of the 

 vessel below deck. Admiral Labrousse has produced a modified form of the 

 Moncrieff gun-carriage, substituting springs for the counterpoise, which has 

 been tried with successful results. A corps of Engineers has been formed under 

 the direction of M. Tresca, Director of the Conservatoire des Arts et Metiers, 

 to form what are called casemented shields in all the sections of the fortifi- 

 cations, for which purpose a sum equal to ,£"9000 has been granted. In the 

 forts round Paris there were, until quite recently, neither abris, platforms, maga- 

 zines, casemates, embrasures, nor any of the necessary accessory works without. 

 All these were, however, rapidly supplied by the military Engineers. Besides 

 such works, the sixty-nine gates of the enciente had to be closed and furnished 

 with bridges. Four canals had to be barred, and the Seine stockaded. The 

 military zone had to be cleared of houses, portions of the Bois de Boulogne 

 and Vincennes cut down, the forts supplied with palisades, extending in all to 

 61,000 metres, and, finally, three new batteries were formed at St. Ouen, 

 Montmatre, and the Buttes Chaumont. In 1840 the armament of Paris was fixed 

 at 1824 pieces, with a reserve of 540 ; now there are 2igo guns in battery, with 

 a reserve of 350. The bronze field guns cast in Paris during the siege are 

 composed of a mixture of go parts of copper and 10 of tin ; the lining of brass 

 consists of about 62 parts of copper, 35 of zinc, and 3 parts of tin and lead. 



Balloon Navigation.— Since the investment of Paris the only communication 

 with the outer world has been carried on by means of balloons. Almost as 

 soon as these aerial carriages were invented they became applied to the 



