SCIENTIFIC SUMMARY. 
11 ? 
chalk ; so that it is scarcely possible to resist the inference that these great 
masses of grit are connected with and probably derived from that deposit. 
As, however, a similar sandstone is found at some places in the Woolwich 
and Reading strata, which underlie the London clay, it is not unlikely 
that some may also have come from those beds. 
The unknowable is ever becoming a more narrow field, and probably 
the exact thickness of the earth’s crust will soon be as well known as our 
planet’s diameter. Not many years since, it was generally fixed at ten or 
fifteen miles ; then Mr. Hopkins demonstrated, on astronomical evidence, 
that it could not be less than 800 miles; and, now, Professor W. Thompson 
has pointed out a means by which a more exact measurement may he made. 
His argument appears to be that the earth is extremely rigid, for were its 
entire mass of the rigidity of glass it would yield 7-0ths of the extent to 
which a perfectly-fluid globe of the same density would be influenced by 
the lunar and solar tide-generating power ; and were it as rigid as steel, 
it would yet yield 2-5ths of that amount. Hence the actual phenomena of 
tides would, in the former case, be only 2-9ths, and in the latter, 
but 3-oths of the amount which a perfectly rigid spheroid of the same 
dimensions, same figure, and same homogeneous density would exhibit in 
the same circumstances. And therefore, by carefully observing the 
amount of the tides at different stations, it will be possible to determine 
the exact rigidity of the earth, so that, the density being known, it will 
not be difficult to arrive at a near approximate knowledge of the 
thickness of the crust. On this evidence, the Professor inclines to think 
that the fluid part of the earth must be at a depth of from 2,000 to 2,500 
MPORTANT experiments on marine screw-propulsion have recently 
been made in II.M.S. Shannon. The experiments in the Rattler 
and Minx , in 1843-5, led to the adoption of what is now known as the 
common screw, and which, in spite of some defects, is extensively em- 
ployed to the present time. It consists of two “ blades,” broadest at their 
periphery and tapering to their junction with the propeller shaft. It is 
defective in this — that the root ends of the blades (nearest the shaft), being 
twisted nearly perpendicular to the plane of revolution, cause centrifugal 
action in the water. The power uselessly expended in this way amounts 
in some cases to 20 per cent, of the whole work of the engines. To 
remedy this, in the Griffiths screw, one of the best known of numberless 
attempts at improvement, the blades are fixed on a large spherical boss, 
about one-third the diameter of the propeller. Thus a sufficient surface is 
afforded for the attachment of the blades without twisting them into line 
with the shaft so as to waste power in centrifugal action. The blades are 
usually three in number, to diminish vibration, and are broadest at their 
roots, tapering outwards. In the French navy another form prevails, 
known as the Mangin screw, and consisting of four blades set in pairs 
parallel to each other, at a short distance apart. The object of the recent 
experiments in the Shannon has been the determination of the merits of 
these three forms of propeller. 
miles. 
MECHANICAL SCIENCE. 
