a m i O D U C T I O N.
1 1 II I
be the- earth’ s axis, i?S^'thé. equator, j?», jB»i, tw o - parallels
to it, the moondelcribing the parall 1 An. Then
by the earth’s rotation, the places A and B are 'Cdffittl
•from A to rt„ ‘afgjiftjiqi @ to in, and then to t??,
?n 1 fr< in m to B . Hence, the high tides to «thole ;Vfo
places areht A and? B , and the low1 tides at ji and Jm ;
f qdichinSjïlfe dSfeif!e<between th cn C jgh tófjth(-jti,-gh
-and loW tides will be, the 'diffeSencê ofQ^Opid-©«, and
o f OB and Om j and as Om and On, are greater tha l OC,
1 the difference o f the tides, is. lefs here than when theme
n m in tlie-equator. Hence, the tides are highelt
when the moon is, in the equator; and as the riioon recedes
Trom the equator, the tides diminilh. ’
> i S i . Hence, the higheft tides are when the-new or
full Moon Kappens at the time when the fun i ifi th?
equator, or about Maith 22d, and «September the 22d,
for then the-moon, which is in conjunction with or op
polltion to the fun' at thole times, muff alfo be in the
equator. An d if the moon be alfo then at its neareft
diftance, the tides will be the greateft o f all.
, iijShi That^the-.tideis Jtnay have their full effe£fj‘.the
füfface o f the earth oiigfit;.to be covered uith.water.;, and
hence, in large feas thé éffcïf.is.greateft. This is the
reafon that the tides ar.e not fo great in tBejforrid '.zofier
between Africa and America,- where the ocean is narrower,
as in the temperate zones of]-either fide. A n d
- from this we may underhand why the tides are fo fmall
in iflands that are very far dittïnt from Jhores. In the-At-
ilanficQÜiè crater cannot rife on one fliore but by d efending
on the other: fo that, at the intermediate fliftant
’ iflands, -it w ill vatf hut a little.fronTthc mean height.
183. A s the tides pafs over (hoals. and run .through
■ ftraights into ba y s -o f the fea,' their-motion becomes"
, more various, and their heights depend on mahy circnm -
fiances. I t is high water on the coafts’ o f Spain and the
weft ó f Ireland, about 3 hours after the móon has paffed
the meridian. From thence it flows into the adjacent
channels, as it finds the eafieft paffage. - One current
from it,“for example, runs up b y the fouth o f England,.
another comes in b y the north, o f Scotland, T h e y take
a confiderable time to move all this way, and it is high
water fooner in the places to which they firft come: and
it begins to fall at thefe places, ,* \ . 1 they are rifing
. -wither oil}in their ctnirfe. A s they return they are not
able to raife the tide, becaufe 'the water runs fatter off
■ then -it returns, till, by a new. tide from the open ocean,
■ the return «of the current is flopped, and the -water begins
to rife again. The -tide takes j i hours to come from
, the ocean to Bondon,: So that when it is- high water
there, a -new tide is already comejinto the ocean; and in
, fqineintermediate place, if mutt "be;low water at the
fame time. When the tides rLmj' byer .flioals, and flow
upon flat (hores, the water rifes to a gtesttfir Height than
hr the deep and open oceans; , hecaufé thé' force', o f Its-,
motion cannot be broke upon level' ihores, mill the water
rifes to a-great height.
a'84. I f a place communicate with-.two'pecans, dr two
wat'3 with the fame ocean, one b f which is-ajreadier paffage
than thê-ótliér, two tides may arrivé at (that place
-at different times, which interfering with each other,
may produce a variety o f phænomena. A t Bnljha, a
port in the kingdom o f Jrunquin, in ififypa/l Inihe , in
latitude 200. -50' N . the day in which the moon paffes
tflê'éqiîator-, ,_r|cq water ilagnhtes ^
as the moon'removes from the equator, the water begins
to rife nanff Ï&1,’onceia-day, and it is'bigh water'at ,thej ■
letting o f the moon, and low-water at hèr rifing. This
daily tide méreâfesqfor: about y 'Or'S days, and then *de-
c eafe: by thejfame degree fo the lame ime, till the
motior ceafes at the moon’ s return 1 1 time equ tor.
\fHtu*Hct ha paffed the eqgaepr. and declines fouth-
waid, theewater rifes and falls again jasubefore ; but itjs
higlowit l i no a ■> t* the’ ’^Wie ÎUth.glofi
-the moon, t i ,
' jN jK t.-.S ir I. N ewton thus accounts for.Iliis:ppén^à
thenon- T ilfBnïfM there are tw o inlets, one from the
Chirffe Opèin between the .Continent and the'
the other}frojpjihe jmfiiîàyÔcean between file Continent
- and ^Bornèofi and he fiipgoffis .that a. tide may .arrivent .
Batjha, ttrojîgmpne^of thèfe iplet j a l'th e f mrd hour o f -
the ;mo:on, and the other through the other 'inlét fi .hours
âfteri For whil ffelHc fe I des are equal,' the cine -flov ing
out P"Mie othtejflqw . in,'thewater mutt itagnatc.’ {Shew
~ffib v v are riff alnvjnaiilm- .ngjoair, 5n!.tl j to r u t jy
' when'the móonf-gets on the fame fide o f the equator’paithil
n^STj^Lb f t o K l t w o ?
gïéatpr’ .spiel* turn ‘1*1 »^tmeCmufl. 1 i IJ c L ’attTibc'fu fib f.
t t f r h % .Tqè é ro d é e an -
of the wate I ,'wBlchM Jt the f
if llâ ^ î^ S hcwylfn c i l - lidl„, j_'dmil Î A K
atlKeiméanjtime Istwêeri'tne'f'wo l^àlbt^raljJCTiàf it;,
wil!fie nlgB.water,abpufftlie f i x t h E q f i t t i n g
-Otehe' 'imwn,’ ctçtïlow ^ater-'aw^æ rîBfe? >®|ienölh e r
nf00n’ f Msi onj tire othe t f:de Tjf ’the equatorjfee nightly
title «îll ixcced ihe'd-’ ilj'fflnS1 lüWÜotpwKÆBicIvti&f;
will 6e('a t IffipTlfing, aridjtne jm ^ ^ e ’^ th e fe t t in g } of ^
theaidoni^ T lie fame jwiricqueri-.wfll-aqcqunt for
cxh.vmfrclinary'i'idu-sriylucl-. ire obfervédl'
-' i S'öijThere arèjno tidesfin'lakes, becaufe* they arr'geoc-
raffÿ -
eqfi^my,ahp'thett-efore fid.jpairlof jxhe water is râiftff *
allow? the other. T h e Starter cancan ^ and 'Baltic .Sea’s
have v ery, fmall t ides, h ecâufé■ tl)e;,iiiletsifiy' which they
c 0hirn h rji Ca thwi t'n d t hat' |
not, in .fpfhort a time, - receive or dlfchàrge. enpiigli to
raife or;fink thar ffürfàeesiîenfibly. In the Mediterranean, .
the;tides produce'a' yariatiph o f about 1 -foot in the
, height o f the waters.
, Toffiyd the I,ongitk'A.e of PJaces upon the
\ j .g E a r th ’j , Surface.', m- -,
" T87. T he 'fitiiation' o f a. place upon the. fuhface b f the
earth, is -determined from its lat-itiide arid, longitude. -
The methods o f finding the. ■ ■ latitude we hav.eitolïeady '-
explained.; ' hut the. ïongitûdc Saqhot Be ' fo readily
m ■ - g ) BB M N D ■ B HHj wi ■ ■ ■ M | '.-, . i. found*
■ ill
ill I
i Ê f à ï 9 * i i Q i N . XXXIX
foiitid.*' Philip I I I . king o f Spain, was the fiiUlgêrfon
who offered a' reward for its difeovery ; and theßätes of
Holland foon after^foHowe'd his example. * '®uring Jibe
promifedsä grfat 'ïeward Hto;^n^;perfon who fhould dif-
1 cojrcr the longitude at fea'.. In the t j j l l g f o^ C haules
I I . the Sieur de St. P ie r r e , a FraVd'Ma-n, pröpofed a-
end j n
this, a commiflion. was granted to Lord Vifcount
to I reegive |ai^p|dpp’w 4v^ d
giv;e their’ppinions r-efpeélrng it. Mr. F lamstead g-aVe
,his o-pmibiiiji thut i f we liad the places ,of ]'tÉe\êxed (lars,
and tables of the moon’ s motion, we might lind the lon-
_giku ut'pipiiby^fehe me t h o ' d l the-ßieur de St.
P i err | fb , F la m s tead was appointed
M o n om e r royal, and,a«n .obfeiwatory was built'at Greenwich,
for him ; arfd ,th<||nIJmSMHs|tö him and his-fpe-
ceflors were, u that MtàiéÊmikM. with- :
/the ntmoft, care an^diligence, to reâiify tire tabks^ohthe
motions of the heayens,, and the plàces’of the fixed ftais,
in ordier to F n p lB : th e^ rpu ch defired longitude at fea I
for the, perfecting o f the A r t o f Navigation,” - /
i ^ i ’88. In' the y,ear^714, the Britifi pailiamtnt offered
a reward^for the difeovery of-the longitude ; the fum >qf
I oçqo /. i f the method' d'e#nmhçdâthe longitude to i°
o f "a great circle,-or to 6ó| gepgraphical miles ; of
:it ■ M d 20i^0^|
i f it'determined it 'tq 30ymiles ; with this provifo, that
if any' -fuck method ex£gn|hno further than 30, miles ad-
joining to1 the cqah, the propofer fh|Mc|have no more
r-fwards'.^^ theÆQi;
Lord o ffflS I Admiralty, the Speaker o f t-he'H© u f< o f
C o m m o n 's f i i Cömmißioner öf Trade, the Admirals
o f the Red, Wliite and Blue Squaïâ-nsV the Matter
'o f Ternit y HouCeMf^ Pre|||nJ of the Royal':;ß©piety,|
the Royal Aftronomer at Greenwich, the two Savilian
jrofeffqrs' H 1 .the;^ ^ a l j | i ; ^ a n 'P1 ürnîaja
.Profettois lat Cambridge^ witfcr feveial other peifons, as'
.Cpmmittioners for .the Longitudb at^ Sea. The-Lown-
dian Profettor at ''’Cambridge" - was * afterwards
A f te r this a<T o f parliament, Several other àffs pàfîed in
the-reigns of G eorge I I . and I I I . for the cnfcéùrage- ■
ment o f finding the longitude. A t latt, 111 i>]74,van?
aft paffed, repealing jafl other ads, and^ÿering fepaiate
rewards to any perfon who fliould -difeover the longitude,
either b y the watch keeping tiue time wi.hin cer-
• tain limits, Or' by 3 |gj lunar method, or by other
rrieansv The;:a£t proj)pfes as a regard'for a tiine-ke’eper,
the fum op£oood. i f it d etè fm in ^ fh è^ w ^ d fj^m '^% -
60 geographical milea ; the fum, of I p p l /. if it deter- •
pine it to )to miles,; an’d-the fum o f loopp L i f it determine
it to 30 miles, after g^ojpçr friais fpecified in the
ad.' I f the method be by improved, folar and lunar
fables,,,çonttruded upon S ir I. N e k t o n ' s Theory o f
gravitation, th.e author; fliàll he entitled tm ,^006%
diftance of'- the moon from
the Mi^ah^ttais, within ‘fifteen féconds o f a degree,- an-
jlwermg. to about feven minutes o f longitude, after allowing
half a degree for the errors o f qbfervation. And for
any-other niethod, the lame.rewards are offered- as thofe
for timekeepers, provided1; it gives ,û V ’ V i r .true
, within fame limits, and be-pi-afti'cable at fea. The com-.
. niiffioners have .alfo a power o f giving fin aller rewards,
as they (hall ju d g e ,proper, to any one who- {hall make
any difepyery for finding the longitude at fea, though,
not within the jäbove limits. Provided however, that
i f ill; h'perfon oi perfons fh;ill aftc rwards make . u-!
ther difeovery - .as-'to come lyithin .’the above mentioned
limits, fucli fum or fum's as .tlie^yVnaÿ^lhave 1 received,,
{hall be conficlered as part o f fucK greater reward, and
18c;. A lte i the- deceafe o f M r. F c im it e a d , D r.,
H a LI Hi, who wùs; appointed to fucceed him, made a
'feries o f obfervations on .the moon’ s tranfit over the me-
! dian, f ir a coinj lc£ A j ol ti m o f the moon’ s apogee,,
.which-.qh'IeCTations'. being reompared with;the coniputa^
■ fions from the tablesT.;then extant, he'-was enabled tpi
cnm’ iu.tlic tables .of thé moon’ s -motions. An d as Mr.
H adl sy had then inventedlaninftyumentiby.which the
altitudes and di'ftances of-t'he heavenly, bodies? could ; betaken
at fea, D r . H a l l e y ; flfrcingly i tcômnoefiàfed'^i«.-
’ lhnfeîq|£ht^îpf t dlauîg}t:H d rdn gi t ti d e.
fe; find, jfie] Lot$it%de^}m,Afftme- keeps-f.
Æ^WTOSettdn appears to move roupdfthe earth from
eaft to weft;, or to deferibe 36 B in 24 hours, and there-
•fbre 1 e appears to move 15° -indait hour i f therefore the
mericlians o f twbj places, make, ah; angle 'of 15° with each,
other, or -iff the two places differ iy ° in longitude, the
'frin; ‘will.'come to ' thereafter^ mm'diani l hour before he
-comes td the wettern; meridian; and therefore when it is 1 2
'b’clbck;at thé former place, it is only eleven at the latter :
and in general, th e .differencc between the times by,;thé
iëlôek at any; two places, will fce-'-the difference o f their
.longitudes; converted .'into rime at the rate o f 15° for'an-
hour, the 1 r 1 ' eàflçrn placé being theforwardeft.
I f therefore we ean tell what o’ clock-it is a t' any two.
places, at the'fame inftant of time,-we .can find the difference
o f their longitudes,, b y allowing*^?0’for every
hour that the clocks differ.
* In many of the old maps,«the-fifft nieridian :is mads-'tö. pàfs. H ® 5 mil s Canal ïes, which ; is. 170. ^ 5'. 50" weft of Greenwich.
-To reducectherefóie jhe'longitude fromïEcrrwtn that frpm'i G'rèehwich, aitel 17 0 45.-' 50" if ehe place be :.-vr/i of Ferro, and it gives the tongi-
t\uie weft from G r e .n w ic h if the place be eo^jof'Ferro,.-"and in longicude;.7^ Tthan -170- 4.5'. ;o". the difference o/i-its longitude, and 17°.-
45'.' 5ok,;:fliowslihe.longitucle\ve({;fiio’in GVeeiiW!ch;sbuc ii thejlo'ngitdde.hé .grepttr than 17°. 4:;-J 50", the ■ difference; fiiows the longitude-
call of Greenwich. T-hus you may reduce the longitude from one place to that from any other. ■