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the Royal Society ; and is d-fcribed in the Phil. Tranf. 

 N" 405. A more particular dcfcription of this inftrument 

 bv Dr. Hales himft.lf is as follows. 



'Suppofe A B {Mljhilnny, Phile I. fs- !•), to be an 

 iron tube, or mudvct-barrcl, of any length, as fifty inches, 

 having its upper end A well dofcd ; if this tube be let 

 down in this pofition about thirty-three feet into the fea, a 

 column of water of that height is nearly equal to the mean 

 weight of our atmofphere, and, confequeiitly, from a known 

 property of the air's elallicity, it will be compreffed into 

 half the fpacc it took up before, fo that the water will alcend 

 half way up the tube ; and if the tube be let down thirty- 

 three feet deeper, the air will be covpreflTed into | of its 

 firll dimenfions, and fo on i, j, i, &c. the air being con- 

 ftantly comprefTible in proportion to the incumbent weight ; 

 whence by knowing to what height the water has afcended 

 in the tube, we may readily know to what depth the tube 

 has defcendcd into the fea. 



Now to nieafure the depth of one of thefe columns of 

 fea-water : firft, by a line let the iron tube, with a weight 

 at its bottom, fink about thirty-three feet, which depth in 

 fait water will nearly anfwer to the weight of the air at a 

 mean height of the barometer ; then draw up the tube, and 

 obferve how far the water rofe. If thirty-three feet of 

 water be equal to one atmofphere, then will the water rife 

 fo high as to fill exactly one half of the tube. But if the 

 water rife higher or lower than half-way, then, by the rule 

 of three, fay, as the number to which the water rifes is to 

 one, fo is thirty-three to the number of feet, meafuring the 

 depth of the column required. For example, fuppofe the 

 water rifes, wlien the tube is let down thirty-three feet, 

 only -i?o of half-way, then fay, 9 : 10 : : 33 : 36^ feet 

 the depth of each column, which being once known, the 

 number of columns of water is to be multiplied by this 

 number of feet, whereby the depth of the fea in feet will 

 be known. 



But fince, when the inftrament has defcended to the depth 

 of 99 columns, or 99 times 33 feet, the air will be com- 

 preffed into the -j.^ part of ^o inches, that is, into half an 

 inch, the divifions both for fome fpace below and alfo above 

 that will be fo very fmall, that the difference in depth of 

 feveral columns of water will not be fenfible. So that an 

 inftrument of no greater length than this vvould fcarcely 

 give an accurate ellimate of half a mile's depth, that is 

 2640 feet, or 80 columns depth of water. The lengthening 

 of this inftrument to 4, 5, or 10 times this length would 

 obviate this defeft, and make the difference of the degrees 

 of defcent much more fenfible. But fince it is imprafti- 

 cable to make a metalline tube of fo great a length, and 

 if it were made, it would be fo unweildy as to be eafily 

 broken, the difficulty may be obviated in the following 

 manner. 



Let there be a globofe metalline body of iron or copper, 

 nearly of this form [Jig. 2.) K, L, M, N, Q, whofe 

 capacity within may be equal to nine times the capacity of 

 the metaUine tube, Z, K, L ; let this globofe body be 

 firmly fcrewed to the metalline tube, at K, L, with a leathern 

 collar, well foaked in fome uniluous matter at the (houlder, 

 or joining, thereby to fecure that joint in the moft effcftual 

 manner. Let there be a fmall hole at X for the fea-water 

 to enter freely in, and let fome coloured oil be poured into 

 the globofe body, to fill it up to the hole X. Let there be 

 alfo provided a (lender rod d h, fcrewed, or faftened into the 

 metalline tube s s, which muft alfo be made to fcrew in and 

 out, thereby to take out the rod at pleafure ; the rod muft 

 alfo have a fmall button d iaftcned to its upper end, which 

 7 



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will prevent its being daubed by falling againfl the fides of 

 the tube. 



The capacity of the tube muft be eftimated by pouring 

 water in, when the rod and metalline tube arc fixed in their 

 places. 



Now fince the lower veffcl is fupnofed to contain nine 

 times as much air as the tube Z L, which is the fame thing 

 as if the tube were niiit; times as long, therefore the air in 

 the globofe veffel will not all be forced within the capacity 

 of the tube, till the veffel has defcendcd to tiie depth of 

 nine columns, . r nine times 33 feet ; for then the air will 

 be compreffed within one tenth of the fpace it firft took up. 

 Huppofing, therefore, the inftrument to have defcended to the 

 depth of 99 columns of water, or 99 times 33 feet^3267 : 

 then the air will be compreffed within -^-J^ part of 500 

 inches (the capacity of the whole veffel being fuppofed 

 equal to the tube of that length), that is, within five inches 

 of the top of the tube ; and, confequently, the rod d b 

 will be found tinged with the oil, within five inches of its 

 top. 



Suppofe again the inftrument to have defcended to the 

 depth of 199 columns, of 33 feet each, thentheair will be 

 comprefi'ed within -^)^ part of the whole, that is, nearly 

 within 2 J inches of the top of the tube. In this cafe, tlie 

 inftrument will have defcended 6567 feet; that is, one mile 

 and a quarter, and 132 feet. 



Suppofe again the inftrument to have defcended to the 

 depth of 399 columns, then the air will be compreffed into 

 •5-i'j^ part of the whole, that is, nearly within one inch and 

 a quarter of the top of the tube. In this cafe the inftru- 

 ment will have defcended two miles and a half, wanting 53 

 feet, which may probably be the greateft depth of the 

 fea. 



The larger the capacity of the veffel K, L, M, N, Q^, 

 the deeper will the gage be enabled to fink, the inftrument' 

 being made ilronger, and its joints being fecured in pro- 

 portion. 



The inftrument being thus prepared, a large buoy, /, 

 muft be fixed to it, which ought to be a folid piece of light 

 wood, well tarred to prevent the water's being preffed into 

 the fap-veffels ; and as it may rife at a confiderable diftance 

 from the fliip, it. may be advifable to fix on the top of the 

 buoy broad fans of tin, properly painted, fo as to be eafily 

 feen. 



In order to fink the inftrument, a weight muft be fixed to 

 it in the following manner. See the diagram. W is a weight 

 of ballaft, hanging by its flieet fhank T, in the fockety^^ 

 which focket is fcrewed faft to N Q^ The (hank is retained 

 in its place by the ketch k of the fpring O, while the ma- 

 chine is defcending ; but as foon as W touches the ground 

 at the bottom of the fea, the ketch O k finking by the de- 

 fcending force, a httle below the upper part of the hole /■, 

 is therefore at liberty to fly back, and fo lets go the weight ; 

 then the buoy rifes up to the furface of the water with the 

 machine. Springs might alfo be fixed on the infide of the 

 focket _/"y", fo as to fly back in the fame manner, when the 

 weight touches the ground. It might be advifable to keep 

 an exaft account of the ftay of the machine under water, 

 which might be done by a watch, or by a pendulum vibrat- 

 ing feconds. Dr. Hook found upon trial, that a leaden 

 ball which weighed two pounds, fixed to a wooden ball of 

 the fame weight,and both let down in fourteen fathom water, 

 reached the bottom in feventeen feconds, and the detached 

 wooden ball afcended to the furface in feventeen more. 

 See Phil. Tranf. Lowthorp's Abr. vol. i. p. 258. Confe- 

 quently if this machine defcended and afcended greater depths 



with 



