a city of Kent, in England, is situated on the Medway, seven miles and a half north of Maidstone, and 30 from London. It appears to have been one of the Roman stations, from the bricks in the walls, as well as the Roman coins that have been found about it. It has three parish churches built with stone and flints, besides the cathedral, which is but a mean structure. This little city, which was made a bishop's see by King Ethelbert, anno 604, has met with many misfortunes. In 676, it was sacked by Eldred king of Mercia; in 839 and 885, besieged by the Danes, but refused by King Alfred. About 100 years after, it was besieged by King Ethelred, and forced to Rochester, pay 100l. Anno 999 it was taken and plundered by the Danes. Anno 1088 it was besieged and taken by William Rufus. In King John's time it was taken from the Barons, after three months' siege; and the very next year, viz. 1256, its castle, founded by William the Conqueror, was stormed and taken by several of the Barons, under the French king's son. In the reign of Henry III. it was besieged by Simon Montford, who burnt its then wooden bridge and tower, and spoiled the church and priory, and then marched off. This city has also been several times destroyed by fire, viz. in 1130, on June 3. in 1371, and in 1177; after which it is said to have continued desolate till 1225, when it was repaired, ditched, and walled round. In the Saxon heptarchy there were three mints in Rochester, two for the king and one for the bishop. In 1281, its old wooden bridge was carried off by the ice, in a sudden thaw after a frost which had made the Medway passable on foot. Another was built in the reign of Richard II. but pulled down again, on the rumour of an invasion from France. It was afterwards restored, but so often subject to expensive repairs, by reason of the rapid course of the river under it, as well as the great breadth and depth of it, that in the reign of Edward III. it was resolved to build a new bridge of stone; and the same was begun, and in a manner completed, at the expense of Sir John Cobham and Sir Robert Knolles, Edward III.'s generals, out of the spoils they had taken in France. It has 21 arches. The town is governed by a mayor, recorder, 12 aldermen, 12 common-councilmen, a town-clerk, three sergeants-at-mace, and a water-bailiff. To its cathedral belong a dean and six prebendaries. Gundulph's tower stands on the north side of the cathedral, and is supposed to have been built by the bishop, as a place of security for the treasures and archives of that church and see. Some suppose it to have been intended for a bell tower, and others for an ecclesiastical prison; but whatever might be its destination, its machicolations, its loop-hole windows, and the thickness of its walls, show that strength and defence were considered as necessary. This tower was 60 feet high, but some part has lately fallen down; the walls are six feet thick, and contain within them an area of 20 feet square; it was divided into five floors or stories of unequal height, and had a communication with the upper part of the church, by means of an arch or bridge, the steps of which are still visible. It is supposed to have been erected after the cathedral was built. For the maintenance of its bridge, certain lands are tied down by parliament, to which it has sent members from the first. The town-house, built in the year 1687, for the courts, assizes, and sessions, and the charity-school, are two of the best public buildings here.—A mathematical school was founded here, and an almshouse for lodging six poor travellers every night, and allowing them 4d. in the morning when they depart, except persons contiguously diseased, rogues, and profilers. In the summer here are always six or eight lodgers, who are admitted by tickets from the mayor. The Roman Watling-street runs through this town from Shooter's Hill to Dover. The mayor and citizens hold what is called an admiralty-court once a-year for regulating the oyster-fishery in the creeks and branches of the Medway that are within their jurisdiction, Conceive the rocket to have no vent at the choak, and to be set on fire in the conical bore; the consequence will be, either that the rocket would burst in the weakest place, or, if all its parts were equally strong, and able to sustain the impulse of the flame, the rocket would burn out immoveable. Now, as the force of the flame is equable, suppose its action downwards, or that upwards, sufficient to lift 40 pounds. As these forces are equal, but their directions contrary, they will destroy each other's action.

Imagine then the rocket opened at the choak; by this means the action of the flame downwards is taken away, and there remains a force equal to 40 pounds acting upwards, to carry up the rocket, and the stick it is tied to. Accordingly, we find that if the composition of the rocket be very weak, so as not to give an impulse greater than the weight of the rocket and stick, it does not rise at all; or if the composition be slow, so that a small part of it only kindles at first, the rocket will not rise.

The stick serves to keep it perpendicular; for if the rocket should begin to stumble, moving round a point in the choak, as being the common centre of gravity of rocket and stick, there would be so much friction against the air by the stick between the centre and the point, and the point would beat against the air with so much velocity, that the friction of the medium would restore it to its perpendicularity.

When the composition is burnt out, and the impulse upwards has ceased, the common centre of gravity is brought lower towards the middle of the stick; by which means the velocity of the point of the stick is decreased, and that of the point of the rocket increased; so that the whole will tumble down, with the rocket-end foremost.

All the while the rocket burns, the common centre of gravity is shifting and getting downwards, and still the faster and the lower as the stick is the lighter, so that it sometimes begins to tumble before it be burnt out; but when the stick is a little too heavy, the weight of the rocket bearing a less proportion to that of the stick, the common centre of gravity will not get so low but that the rocket will rise straight, though not so fast.