SCIENTIFIC SUMMARY. 
663 
MECHANICAL SCIENCE. 
Wire Bopes . — Mr. G. C. Greeuwell, F.G.S., has communicated a valuable 
practical paper on the use of wire ropes in collieries to the Manchester Geological 
Society. He concludes that round wire ropes are very superior in economy and 
durability to the flat ropes first introduced, and still generally employed. The 
maximum safe working strain is given at 5 cwt. for each pound weight per 
fathom of the rope, if round, and 4 cwt. if flat. The durability is stated to 
be from one to two years, not being much affected by the load raised daily. 
Steel ropes are suggested for great depths, and the balancing of the puUey 
at the pit head on inverted coach-springs, so as to relieve the rope "from 
sudden strains. 
Ordnance and Targets . — The Admiralty having erected a new target, repre- 
senting a portion of the side of the Hercules, experiments were made at 
Shoeburyness which proved that a thickness of armour casing had been 
attained which afforded perfect security against even the largest guns recently 
constructed. The target has a facing of 9-inch armour-plates, and contains 
altogether 11 inches thickness of iron. Against this, three 12-ton shunt guns 
were fired, at a distance of only 200 yards, with charges varying from 
451b. to 601b. of powder. One steel shot, of 3001b. weight, 10^ inches in 
diameter, fired with 601b. of powder, at a velocity of 1,450 feet per second, 
barely broke through the armour, without injuring the backing. Sir William 
Armstrong has expressed his conviction, in the Times, that the 600-pounder 
gun will be unable to penetrate this target, and that it will, in fact, require 
a gun carrying 1201b. of powder and steel shot to pierce this massive shield. 
Mr. W. C. Unwin has pointed out, in a letter to the Engineer, that for 
similar guns, with shot of similar form, and charges in a constant ratio to the 
weight of the shot, the velocity is nearly constant. Then, assuming the 
resistance of the plates to be as the squares of their thicknesses, it follows 
that when the diameter of the shot increases, as well as the thickness of the 
armour, the maximum thickness perforated wiU (by theory) vary as the 
cube root of the weight of the shot, or, in other words, as the calibre of the 
gun ; and the weight of the shot necessary to penetrate difterent thicknesses 
of armour will be as the cubes of those thicknesses. The ratio deduced from 
the Shoeburyness experiments is somewhat less than this, being as the -f-th 
power and the -|nd power respectively. Practical formulae deduced from 
experiments are given, which agree with Sir William Armstrong’s conclusion, 
and prove that a gun which can effectively burn a charge of at least 100 lb. 
of powder will be required to effectually penetrate the side of the 
Hercules. 
The Friction-hreah. — Mr. Charles T. Porter has made some careful experi- 
ments, in which the power of a steam-engine was simultaneously measured 
by the friction-break and the indicator. They show so singular a discrepancy, 
that he has concluded the friction-break to be a fallacious test of engine- 
power. No satisfactory explanation of the difference has yet been suggested. 
The power calculated from the weight on the break is greater than that 
shown by the indicator. 
