Encyclopaedia Britannica, or, a Dictionary of arts, sciences, and miscellaneous literature : enlarged and improved. Illustrated with nearly six hundred engravings > Volume 18, RHI-SCR

ROC [ i°3 1 ROC
Rocliefler tion, and for profecuting the cable-hangers, as they are
It called, who dredge and fifli for oyfters without being
, llotket- free, by having ferved feven years apprenticefliip to a
fi(Herman who is free of the fifhery. Every licenfed
dredger pays 6s. 8d. a-year to the fupport of the courts,
and the fifhery is now in a flouriiliing way. Part of the
cattle is kept in repair, and is ufed as a magazine, where
a party of foldiers do conltant duty. 1 he bridge was re¬
paired in 1744, ^nd pallifadoed with new iron rails. Ro-
chefter contains about 700 houfes, and 2000 inhabitants.
It confiits of only one principal ftreet, which is wide,
and paved with flints. The houfes are generally well
built with brick, and inhabited by tradefmen and inn¬
keepers. It has alfo four narrow (treets *, but no fort
of manufactory is carried on here. Stroud is at the
weft end of this place, and Chatham at the eaft. It is
27 miles north-weft by weft of Canterbury, and 30
fouth-eaft by eaft of London. Long. o. 36. E. Lat. 51.
23. N.
Rochester, Earl of. See Wilmot.
ROCK, a large mafs of (tone. See Geoeogy.
Rock, a fpecies of Vulture. See Ornithology
Index.
Rock Bafons are cavities or artificial bafons of diffe¬
rent fizes, from fix feet to a few inches diameter, cut
in the furface of the rocks for the purpofe, as is fup-
pofed, of collecting the dew and rain pure as it de-
fcended from the heavens, for the ufe of ablutions and
purifications, prefcribed in the druidical religion } thefe,
efpecially the dew, being deemed the pureft of all
fluids. There are two forts of thefe bafons, one with
lips or communications between the different bafons,
the other fimple cavities. The lips as low as the bot¬
tom of the bafons, which are horizontal, and commu¬
nicate with one fomewhat lower, fo contrived that the
contents fell by a gradual defcent through a fucceflion of
bafons either to the ground, or into a veffel fet to re¬
ceive it. The bafons without lips might be intended
for refervoirs to preferve the rain or dew in its original
purity without touching any other veffel, and was per¬
haps ufed for the druid to drink, or wa(h his hands,
previous to officiating at any high ceremony, or elfe to
mix with their mifletoe.
Some of thefe bafons are fo formed as to receive the
head and part of the human body ; one of this kind is
found on a rock called King Arthur’s Bed, in the pariih
of North Hall in Cornwall, rvhere are alfo others, call¬
ed by the country people Arthur’s troughs, in which
they fay he ufed to feed his dogs.
ROCK-Crystal, in Natural HiJIsnj, otherwife called
fprig-crystal, a^ame given to quartz or filiceous (tones,
when pure and regularly cryftallized. See Minera¬
logy Index.
Rock Salt. See Salt, Geology.
Rock Oil. See Petroleum, Mineralogy Index.
Rock Fifh. See Gobius, Ichthyology Index.
ROCKET, an artificial fire-work, confiding of a
cylindrical cafe of paper, filled with a compofition of
certain combuftible ingredients', which, being tied to a
(tick, mounts into the air, and then burfts. See Pyro-
TECHNY.
Theory of the Flight of Sky-ROCKETS. Mariotte
takes the rife of rockets to be owing to the impulfe or
refiftance of the air againft the flame. Dr Defaguliers
accounts for it otherwife.
Conceive the rocket to have no vent at the choak, Rocket,
and to be fet on fire in the conical bore j the confe-
quence will be, cither that the rocket would burft in the , ^
weakeft place, or, if all its parts were equally ftrong,
and able to fuftain the impulfe of the flame, the rocket
would burn out immoveable. Now, as the force of
the flame is equable, fuppofe its a&ion downwards, or
that upwards, fufficient to lift 40 pounds. As thefe
forces are, equal, but their dire&ions contrary, they will
deftroy each other’s adlion.
Imagine then the rocket opened at the choak ; by
this means the adlion of the flame downwards is taken
away, and there remains a force equal to 40 pounds
adling upwards, to carry up the rocket, and the ftick it
is tied to. Accordingly, we find that if the compofition
of the rocket be very weak, fo as not to give an impulfc
greater than the weight of the rocket and ftick, it does
not rife at all 5 or if the compofition be flow, fo that a
fmall part of it only kindles at firft, the rocket will not
rife.
The ftick ferves to keep it perpendicular 5 for if the
rocket ftiould begin to Humble, moving round a point in
the choak, as being the common centre of gravity of
rocket and ftick, there would be fo much fridlion againft
the air by the ftick between the centre and the point,
and the point would beat againft the air with fo much
velocity, that the fridtion of the medium would reftore
it to its perpendicularity.
When the compofition is burnt out, and the impulfe
upwards has ceafed, the common centre of gravity is
brought lower towards the middle of the ftick ; by
which means the velocity of the point of the ftick is de-
creafed, and that of the point of the rocket increafcd j
fo that the whole will tumble down, with the rocket-
end foremoft.
All the while the rocket burns, the common centre
of gravity is fhifting and getting downwards, and dill*
the fafter and the lower as the ftick is the lighter, fo
that it fometimes begins to tumble before it be burnt
out; but when the ftick is a little too heavy, the-
weight of the rocket bearing a lefs proportion to that of
the ftick, the common centre of gravity will not get fo
low but that the rocket will rife ftraight, though not fo
faft.
Rocket. See Brassica, Botany Index.
ROCKINGHAM, a town in Northamptonfhire, in
England, 87 miles from London, (lands on the river
Welland. It has a eharity-fchool, a market on Thurf-
day, and a fair on Sept. 8. for five days. Its foreft was
reckoned one of the largeft and richeft of the kingdom,
in which William the Conqueror built a caftle ; it ex¬
tended, in the time of the ancient Britons, alraoft from
the Welland to the Nen, and was noted formerly for
iron-works, great quantities of flags, i. e. the refufe of
the iron-ore, being met with in the adjacent fields. It
extended, according to a furvey in 1641, near 14 miles
in length, from the weft end of Middleton-Woods to
the town of Mansford, and five miles in breadth, from
Brigftock to the Welland ; but is now difmembered into
parcels, by the interpofition of fields and towns, and
is divided into three bailiwicks. In feveral of its woods
a great quantity of charcoal is made of the tops of
trees, of which many waggon-loads are fent every year
to Peterborough. There is a fpacious plain in it called
Rockinghamfliire, which is a common to the four towns
of.