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Show MAE L S T R 0 M. NoTE XXXI. flows into it thrm: gh the Straits; and the ·waters o f t l G lph of Mexico lie much 1C u . . . b I l 1 f ~tl1e rea about the Floridas and farther northward, whtch gtves nfe to a o\'C t 1e eve o 1' ' . the Gul )h-fheam, as defcribcd in note on Cafiia tn Vol. II. . . Ml rn . r. ·u to be fiill twice in about twenty-four hours when the ttde IS The ae nrom ts 1a1 . . ll , and mofl: violent at the oppofite tim es of the day. This IS not ~dficu.lt to acc~unt / fi h r much water is brought over the fubterrancous paflage, 1f fuch cxtfls, 1or, 111ce vv en 10 . h I tl to fill it :md fl:ancl many feet above it, lefs d1fl:urbance muft appear on t e as comp ea y . d'{h A 1 f f f Th Maelflrom is clefcribed in the Mem01re~ of the Swe 1 cac emy o ur ace. e . U · [; 1 M r ~ · Sci· ences, anc1 p ontopl' dd on •s I~Ji fl: . of Norway ' and 111 the mver a u1eum 0 1 1763, P· 1 31• . . b [, I t Tl r why eddies of water become hollow in the middle IS ecau e t Je wa cr Je reatOI1 · [; f · c\:' immediately over the centre of the well, or cavity, falls fafl:er, hav1ng le s n 1011 to 0 ofe its defcent, than the wat r over the circumference or ~dges of the well. The cf:Cular motion or gyration of ecdics depends on the obliqu1ty of the courfe of the iheam, or to the friB:ion or oppofition to it being greater on one fide of the ·well than the other ; I have obferved in water pafiing through a hole in the bottom .of a trough, which was always kept full, the gyration of the {heam might be turned either ~ray by increafing the oppofition of one fide of the eddy wi~h ones finger.' or by turnmg the fpout, through which the water was introduced,. a ltttle more ob.hquely to .the h~le on one fide or 011 the other. Lighter bodie~ are liable to be rctamcd long 1!1 cd~1es of water, while thofe rather heavier than water are foon thrown out beyoncl the CJrcua~ference by their acquired momentum becoming greater than that of the w.ater. Th.us 1f f!qual portions of oil and water be put into a phial, and by means of a ~nn g be \vlml ed in a circle round the hand, the water will always keep at the greater dd1ance from the centre, whence in the eddies formed in rivera during a flood a perfon who en.dr.avours to keep abm·e ·.vater or to fwim is liable to be detained in them, but on fu~e rlllg hmtfelf to fink or dive he is faid readily to efcape. This circulation of water 1n defcct.1ding through a hole in a vefld Dr. Franklin has ingeniouDy applied to the explanat!Oil of hurricanes or eddies of air. { 79 ) NOTE XXXII.----... -GLACIERS. !1/here round darl~ crags iudignaJJt U'attrs bend CJ'brougb r!fted ice, in ivory veins dejcend. CANTO III. I. II 3· THE common heat of the interior parts of the e~rth b · 1 8 d · . " emg a ways 4 egrees both 111 winter and fummer, the fnow which lies in conta:l:, 'th · · 1 · ·' • . VI It IS a ways Ill a thawmg fiate. Hence •n 1ce-houfes th.e external parts of the collection o f 1· ce · 11 · ' IS perpetua y thawmg and thus prefervcs the Internal part of it· fo that it is ff: 1 . ' nece ary to ay up many tons for the prefcrvat1on of one t. on. Hence in Italy. confiderabl e n· vers ha ve t1 1 e1· r !io urce from beneath the eternal gl:.tc1ers, or mountains of fnow and ice. In. our cou_n t,r y when the air in the courfe of a froft cont '111ues a da y or t wo at very neat 32 degrees, t~e common he~t of the earth thaws the ice on its furface, while the ther~ometcr r~mams at the freezmg point. This circumflance is often obfervable in the nmy m~rn1ngs of f1:ring; the thermometer {hall continue at the freezing point, yet all the nme wdl vanJ{h, except that which happens to lie on a bridge, a board, or on a cake of cow-dung, which being thus as it were infulated or cut off from fo free a communication with tl~e common heat of the earth by means of the air under the bridge, or wood, or dung, whtch :ue bad condutlors of heat, continues fome time longer unthawed. Hence when t~e ground is covered thick with fnow, though the frofl: continues, and tne fun docs not Ollne, yet the fnow is obfervccl to dccreafe very fenfibly. For the common heat .of t.he eartl: melts the under furface of it, and the upper one evaporates by its folutwn w the a1r. The great evaporation of ice was obfc::rved by Mr. Boyle, which experiment I repeated fume time ago. Having fufpended a piece of ice by a wire and weighed it with car.: v. ithout touching it with my hand, I hung it out the whole of a clear frofl y night, and found in the morning it had lofl nearly a fifth of its weight. Mr. N. W::dlcrius has fince obfervcd th!lt ice at the time of its congebtion evaporates fafl:cr than water in its fluid form; which may be accounted for from the heat given out at the infbnt of freezing; (Saulfure's Effais fur Hygromct. p. 249.) but this effeCl: is only momentary. Thus the vegetables tha t are covered with fi10w are feldom injureJ; fince, as they lie between the thawing fnow, which has 32 degrees of heat, and the covered earth ·which has 48, they are preferveu in a degree of heat between thefe, viz. in 40 degrees of heat. Whence the roofs on which the rein-deer feed in the northern latitudcs vegetates b.: neath the fnow; (See note on MufChus, Vol. II. ) and hence many Lapland and Alpine plants perifhed through cold in the botanic garden at Upl:1l, for in their native fituati ons, though the cold is much more intenfc, yet at its very commencement they are conn:d deep with fnow, which remains till late in the fprillg. For this faL'l fee Amreni t. Acadcm. Vol. I. No. 48. I11 our climate fuch plants do well covered with dried fern, under which they will grow, anu even flower, till the fc:vcre vernal frofl:s cca[e. For the increa[e of gh .. iers [t;e Note on Canto I. I. 529. |
