GIIAVESEND GRAVITY. 



523 



GRAVESEND ; a market-town of Kent, not fer 

 from the mouth of the Thames, twenty-one miles east 

 of London. It is a great rendezvous for shipping. 

 The numerous vessels which usually lie at anchor in 

 the river, keep up a constant influx of seamen and 

 strangers. The bathing establishment draws addi- 

 tional visiters in the summer season ; and, from all 

 these circumstances, this town presents a continued 

 scene of bustle and activity. There is a canal to 

 Rochester. The inhabitants are much engaged in 

 seafaring employments. A small factory for cables 

 and ropes is also carried on here ; and there is, be- 

 sides, a yard for ship-building, in which several men- 

 of-war have been built. Population in 1831, 5097. 

 During the summer or bathing season, its population 

 is greatly augmented. Population in 1841, 6414. 



GRAVINA, JOHN VINCENT, an eminent jurist and 

 man of letters, was born at Rogiano, a castle in Ca- 

 labria, in 1664. He studied civil and canon law at 

 Naples, and, visiting Rome, resided, for some years, 

 with Paul Coardo, of Turin. He was one of the 

 founders of the academy of the Arcadians, and drew 

 up their laws in the style of the Roman tables. In 

 1698, he was appointed professor of civil law, at the 

 college della Sapienza, and, five years afterwards, he 

 succeeded to the chair of canon law and the exposi- 

 tion of the decretals. He gained great reputation by 

 his writings, which were numerous. The principal, 

 Origines Juris Civilis, is considered a classical work, 

 replete with learning. To the Naples edition, printed 

 in 1713, was subjoined a treatise De Imperio Romano, 

 also highly esteemed. He was also the author of 

 Institutes of Civil and Canon Law ; some treatises; 

 Della Tragedia ; Delia Ragion Poetica ; De Institu- 

 tions Poetarum, and five tragedies, written on the 

 model of the ancients, which were not favourably 

 received. He was invited to Turin by the duke of 

 Savoy, and was preparing to go thither when he was 

 seized with an illness, and died in 1718, in the arms 

 of his scholar, Metastasio, whom he made his chief 

 heir. 



GRAVING ; the act of cleaning a ship's bottom, 

 when she is laid aground, during the recess of the 

 tide. 



GRAVITATION (from gravitas, Latin) ; the act 

 of tending to a centre. Or gravitation may be more 

 generally defined the exercise of gravity, or the action 

 which a body exercises on another by the power of 

 gravity, See Attraction. 



GRAVITY (gravitas, Latin), in physics ; the na- 

 tural tendency or inclination of bodies towards a 

 centre. Terrestrial gravity is that force by which all 

 bodies are continually urged towards the centre of 

 the earth. It is in consequence of this force, that 

 bodies are accelerated in their fall, and, when at rest, 

 that they press the body, or that part of the body, 

 by which they are supported. As to the cause of 

 gravity, or its nature, nothing is known ; and it 

 would be useless to detail the hypotheses advanced to 

 account for this most important law of nature. All that 

 can be said is, that it appears to be an essential pro- 

 perly of matter, or, at least, of all matter that has 

 hitherto become the object of human investigation, 

 though it is by no means certain that matter may not 

 exist, which is not subject to its influence. This part 

 of the subject appears to be beyond human compre- 

 hension. Instead, therefore, of wasting our time in 

 useless speculation as to the cause, let us only attend 

 to its effects, and content ourselves with examining 

 more particularly the manner in which this principle 

 operates on material bodies, and the laws by which it 

 appears to be regulated ; the principal of which, as 

 deduced from experiment, or from the most unequi- 

 vocal inferences, are as follows : 1. that gravitation 

 takes place between the most minute particles of bo- 



dies ; 2. that it is proportional to the masses of thesr. 

 bodies ; 3. that it varies inversely as the square of the 

 distance, in proceeding from the surface of the body 

 outwards, or from its centre ; 4. that it varies direct- 

 ly as the distance, in descending from the surface to 

 the centre in uniform spherical bodies ; 5. that it acts 

 equally on bodies in a state of rest, as on those in 

 motion, and that its action in the latter case is always 

 the same, whether that motion be to or from the 

 centre of attraction, or in any other direction ; 6. 

 that it is transmitted instantaneously from one body 

 to another. Gravity, as relating to the science of 

 mechanics, is divided into absolute and relative. Ab- 

 solute gravity is that by which a body descends freely 

 and perpendicularly in a vacuum or non-resisting 

 medium. Relative gravity is that by which a body 

 descends, when the absolute gravity is constantly 

 counteracted by a uniform, but inferior force, such 

 as in the descent of bodies down inclined planes, or 

 in resisting mediums. (See Inclined Plane.) Specific 

 gravity is the relative gravity of any body or sub- 

 stance, considered with regard to some other body, 

 which is assumed as a standard of comparison ; and 

 this standard, by universal consent and practice, is 

 rain water, on account of its being less subject to va- 

 riation in different circumstances of time, place, &c., 

 than any other body, whether solid or fluid ; and, by a 

 very fortunate coincidence, at least to British philo- 

 sophers, it happens, that a cubic foot of rain water 

 weighs 1000 ounces avoirdupois. Consequently, as- 

 suming this as the specific gravity of rain water, and 

 comparing all other bodies with this, the same num- 

 bers that express he specific gravity of bodies, will 

 at the same time denote the weight of a cubic foot 

 of each in avoirdupois ounces, which is a great con- 

 venience in numerical computations. From the pre- 

 ceding definition, we readily draw the following laws 

 of the specific gravity of bodies, viz. 1. in bodies of 

 equal magnitude, the specific gravities are directly as 

 the weights, or as their densities ; 2. in bodies of the 

 same specific gravities, the weights will be as the 

 magnitudes ; 3. in bodies of equal weights, the spe- 

 cific gravities are inversely as the magnitudes ; 4. 

 the weights of different bodies are to each other in 

 the compound ratio of their magnitudes and specific 

 gravities. Hence, it is obvious, that, of the magni- 

 tude, weight, and specific gravity of a body, any two 

 being given, the third may be found ; and we may 

 thus find the magnitude of bodies, which are too 

 irregular to admit of the application of the common 

 rules of mensuration ; or we may, by knowing the 

 specific gravity and magnitude, find the weight of 

 bodies which are too ponderous to be submitted to 

 the action of the balance or steelyard; or, lastly, the 

 magnitude arid weight being given, we may ascer- 

 tain their specific gravities. 



Other properties relating to the specific gravity of 

 bodies are as follows ; viz. 1. A body immersed in 

 a fluid will sink, if its specific gravity be greater 

 than that of the fluid ; if it be less, the body will 

 rise to the top, and be only partly immerged ; and if 

 the specific gravity of the solid and fluid be equal, 

 it will remain at rest in any part of the fluid in which 

 it may be placed. 2. When a body is heavier than 

 a fluid, it loses as much of its weight, when immersed, 

 as is equal to a quantity of the fluid of the same bulk 

 or magnitude. 3. If the specific gravity of the fluid 

 be greater than that of the body, then the quantity 

 of the fluid displaced by the part immerged, is equal 

 to the weight of the whole body ; and hence, as the 

 specific gravity of the fluid is to that of the body, so 

 is the wnole magnitude of the body to the part 

 immerged. 4. The specific gravities of equal solids, 

 are as their parts immerged in the same fluid. 5. 

 The specific gravities of fluids are as the weights 



