HYD 



HYG 



more acute the triangle in that part, the 

 less the resistance ; for the triangle dis- 

 places only half the quantity of waterthat 

 would be removed by a parallelogram of 

 equal base and altitude ; ergo, it would 

 proceed twice as far, within a given time, 

 as the latter, were not the friction, in some 

 degree, increased. 



It must be obvious, that whether the 

 vessel alone, or the circumstance of her 

 being laden, cause her to weigh more 

 than the quantity of water displatcd by her 

 whole bulk, up to the very gunwale, is not 

 material ; for in such case she cannot float, 

 but must be depressed by the sum of spe- 

 cific gravity thus produced. This will 

 appear in a very natural and simple man- 

 ner, if we load a cup with small shot, &c. 

 for, though partly empty, the cup will sink 

 whenever the whole weight may exceed 

 that of the water displaced- Both the cup 

 and the shot are, however, specifically 

 heavier titan their bulk of water, and the 

 former would sink if let in sideways ; but 

 then it would only displace a quantity of 

 water corresponding with its own bulk, 

 which would be trivial, when compared 

 with that removed by its pressure as a 

 floating body. On the other hand, we 

 find that a ship may be laden with cotton, 

 which is far lighter than water, so as to 

 sink, at least to a level with the water, 

 though not to precipitate to the bottom, 

 unless forced by the adjunction, in what- 

 ever form or manner, of such other sub- 

 stances as are heavierth in vvater,by which 

 the levity of the cotton may not only be 

 counterpoised, but exceeded. In India, 

 where the principles of hydrostatics are 

 absolutely unknown, the peasants make 

 rafts of the straw, which they perceive to 

 be lighter than water, and on them load 

 the corn threshed from that straw, per- 

 ceiving it to be heavier than water. Thus 

 they act upon the best principles, merely 

 from observation ! 



Perhaps, among the most curious cir- 

 cumstances that come within the verge of 

 our subject, nothing can more fully exem- 

 plify what has been advanced, than the 

 fact, well known, of some vessels sailing 

 better upon than before the wind. We 

 have no doubt that, if the forms of their 

 bottoms were correctly ascertained, they 

 would be found to present such a surface 

 in the former position, when " keeled a 

 little," as created a more favourable po- 

 sition of the gravity of the vessel, though 

 it must be at least equal, or, indeed, great- 

 er, if much pressed by the wind, than m 

 the latter position. 



Before we quit this subject, it is neces- 

 VOL. VI. 



sary to inform the reader, that, except in 

 cases relating purely to statics, few in- 

 stances occur, in which the various mat- 

 ters appertaining to hydrostatics can be 

 treated in a manner perfectly abstracted 

 from pneumatics, or from hydrodynamics. 

 Under the head of FLUIDS, and of HY- 

 DRAULICS, we have treated of the princi- 

 ples of fluids in motion, in such a way as 

 may give a popular idea of those very in- 

 tricate subjects; recommending to the 

 student to read the whole contained under 

 those articles with attention, and combin- 

 ing their sevend actions as derived from 

 one great principle. 



HYDRO sulphuret, in chemistry, the 

 combination of sulphuretted hydrogen 

 with an alkaline or earthy base. The 

 general properties of these substances 

 are, that they are soluble in water, and 

 are crystallizable ; the solution is colour- 

 less, while the action of the air is exclud- 

 ed ; but when that is admitted, a yellow 

 colour is soon acquired, owing to the 

 oxygen of the atmosphere combining with 

 the hydrogen of a portion of the sulphu- 

 retted hydrogen, while the sulphur com- 

 bines with the remaining portion of it, 

 forming a super-sulphuretted hydrogen, 

 in union with the base. Mr. Murray ob- 

 serves, that "the knowledge which we 

 have acquired of sulphuretted hydrogen, 

 and of its combinations, has thrown light 

 on the composition of the mineral sulphu- 

 reous waters, and of the changes which 

 they suffer. As sulphur is by itself in- 

 soluble in water, and as frequently no 

 traces of an alkali, by which it might be 

 rendered soluble, could be discovered in 

 them, chemists found it difficult to con- 

 jecture by what means its solution was ef- 

 fected. The discovery of sulphuretted 

 hydrogen, and of its solubility in water, 

 solved the difficulty ; and the mutual ac- 

 tion exerted between it and the oxygen 

 elucidate the changes these waters suffer 

 from exposure to the air." 



HYGROMETER, a machine or instru- 

 ment to measure the degrees of dry ness 

 or moisture of the atmosphere. 



There are divers sorts of hygrometers; 

 for whatever body either swells or shrinks 

 by dryness or moisture is capable of be- 

 ing formed into an hygrometer. Such are 

 woods of most kinds, particularly ash, 

 deal, poplar, &c. Such also is catgut, the 

 beard of a wild oat, &c. 



All bodies that are susceptible of imbib- 

 ing water have a greater or less disposi- 

 tion to unite themselves with that fluid, 

 by the effect of an attraction similar to 

 chemical affinity. If we plunge into wa 



E e 



