338 
POPULAR SCIENCE REVIEW. 
case, the best motor is compressed air ; in the second, water under pressure. 
As in tunnel work there can he no admission of fire, combustion, or the en- 
gendering of steam. Compressed air has been happily employed by 
M. Sommelier for the engines used by him in the tunnel of the Alps. 
Water under pressure was employed by M. Perret in the works of the South 
of France railways, to give a rotatory movement to the rings or circles of M. 
Deschaux, armed with diamond points for cutting the hardest rocks. — Vide 
Foreign Correspondence, Chemical News. 
The Chatham Ballast-iron. — The Admiralty having at last perceived the 
value of the ballast-iron and having determined on its sale, orders for several 
thousands tons of this material have been already sent in. The examination 
of the ballast-iron in use at Chatham dock -yard showed that there are four or 
five different qualities, the best kind being very valuable. The quality of the 
iron, however, can only be ascertained after each pig has been split asunder, 
to allow of the crystals being seen ; and as the method in which the iron is 
broken has been found to materially affect its quality, the duty of breaking 
each of the pigs of the quantity of iron sold will be undertaken by Messrs. 
Kyland, who will be paid at the rate of Is. 6d. per ton for this work, in 
addition to their commission on the sales. The Admiralty order also directs 
the sale of 2,000 tons of the ballast-iron in store at Sheerness Dockyard, on 
the same conditions. The circumstance of the iron requiring to be broken 
before it is sold will render the entire 4,000 tons useless as ballast-iron, 
should the Admiralty terms not be accepted. It is worthy of remark that, 
notwithstanding the comparatively high value of the ballast-iron, the 
experience of the last few years has completely proved that in a dockyard 
like Chatham, in which the traffic of enormous armour-plates, with the 
heavy slabs, plates, and iron beams used in the construction of iron ships is 
going on all at hours of the day, the granite tramways laid down require re- 
newing and repairing at intervals of a few months; while, on the other 
hand, the passage of the heavy weights has no perceptible effect upon those 
portions of the yard where the tramways are laid with pigs of iron. — Vide 
The Artizan , June. 
MICROSCOPY. 
Under this section we have little to record for the past quarter. Mr. 
Stokes has written to the Microscopical Journal on the subject of u slides by 
post,” and as the matter is of interest we shall give a short account of the 
method he proposes to avoid breakage of slides when travelling through the 
post office. He says, cut two narrow slips of card-board, and gum them 
across the slide on each side of the cover, so as to prevent a slide or the side 
of the box from touching the cover ; roll up four or five slides in paper, and 
place them in one of the ordinary postal boxes. The box should be left 
bare and an ordinary parchment label attached to it, by lacing a cord round 
it. On this label the direction should be written and the stamp affixed. 
New Objects for the Microscope. — Mr. Dancer has read a paper before the 
Manchester Philosophical Society, in which he says that the coal ash from 
