iN A V I G A T I O N. 
618 
being 12^33, when the logarithmic tables confift of eight 
places of figures befides the index. 
In an edition of the Seaman’s Kalender, Mr. Bond 
declared, that he had diicovered the longitude by having 
found out the true theory of the magnetic variation ; and, 
to gain credit,to his affertion, he foretold, that at London 
in 1637 there would be no variation of the compafs, and 
from that time it would gradually mcreafe the other way ; 
which happened accordingly. Again, in the Philofo- 
phical TraniaCtions for 1668, N°4o, he publifhed a table 
of the variation for forty-nine years to come. Thus he 
acquired fuch reputation, that his treatife, entitled The 
Longitude Found, was in 1676 publifhed by the fpecial 
command of Charles II. and approved by many celebrated 
mathematicians. It was not long, however, before it met 
with opposition ; and 1678 another treatife, entitled The 
Longitude not Found, made its appearance; and, as Mr. 
Bond’s hypothefis did not in any manner anfwer its 
author’s fanguine expeditions, the affair was undertaken 
by Dr. Halley. The refult of his fpeculation was, that 
the magnetic needle is influenced by four poles ; but this 
wonderful phenomenon feems hitherto to have eluded all 
our refearches. In 1700, however, Dr. Halley publifhed a 
general map, with curve-lines exprefling the paths where 
the magnetic needle had the fame variation ; which was 
received with univerfal applaufe. But, as the pofitions of 
thefe curves vary from time to time, they fhould fre¬ 
quently be corrected by fkilful perfons; as was done in 
1644 and 1756, by Mr. William Mountaine, and Mr. 
James Dodfon, F. R. S. In the Philofophical TranfaCtions 
for 1690, Dr. Halley alfo gave a differtation on the mon- 
foons ; containing many very ufeful obfervations for fuch 
as fail to places fubjeCt to thefe winds. 
After the true principles of the art were fettled by 
Wright, Bond, and Norwood, the authors on navigation 
became fo numerous, that it would be impoffible to enu¬ 
merate them. New improvements were daily made, and 
every thing relative to it was fettled with an accuracy not 
only unknown to former ages, but which would have 
been reckoned utterly impoffible. The earth being found 
to be a fpheroid, and not a perfect fphere, with the 
(horteft diameter palling through the poles, a tradf was 
publifhed in 1741 by the Rev. Dodlor Patrick Murdoch, 
wherein he accommodated Wright’s failing to fuch a 
figure; and Mr. Colin Maclaurin, the fame year, in the 
Philofophical TranfaCtions, N° 461, gave a rule for de¬ 
termining the meridional parts of a fpheroid ; which fpe- 
culation is farther treated of in his book of Fluxions, 
printed at Edinburgh in 1742. 
The principal foreign writers on navigation are Bartho- 
1 jmew Crefcenti, of Rome, in 1607 ; father George Four¬ 
nier, at Paris, in 1633 ; John Baptift Riccioli, at Bologna, 
ih 1661 ; father Millet Dechales, in 1674 and 1677 ; the 
fieur Blondel St. Aubin,in 1671 and 1673; M. Daflier,in 
>583; M. Sauveur, in 1692; Mr. John Bouguer, in 1698 ; 
father Pezenas, in 1733 and 1741 ; and M. Peter Bouguer, 
who, in 1753, publifhed a very elaborate treatife on this 
fubjedl, entitled “Nouveau Traife de Navigation;” in 
which he gives a variation-compafs of his own invention, 
and attempts to reform the log, as he had done in the 
.Memoirs of the Academy of Sciences for 1747. He is 
alfo very particular in determining the lunations more 
accurately than by the common methods, and in defend¬ 
ing the corrections of the dead-reckonings. This book 
was abridged and improved by M. de la Caille, in 
1760. To thefe we may add Don George Juan of Spain, 
in 1757. 
Among the later difeoveries in navigation, that of 
finding the longitude both by lunar obfervations and by 
tiine-keepers is the principal. It is owing chiefly to the 
rewards offered by the Britifh parliament that this has 
attained the prefent degree of perfection. We are in¬ 
debted to the late Dr. Malkelyne for putting the fint of 
thefe methods in practice, and for other important im¬ 
provements in navigation. The time-keepers, conltruCt- 
i 
ed by Harrifon for this exprefs purpofe, were found to 
anfwer fo well, that he obtained the parliamentary reward. 
See the article Horology, vol. x. p. 346-367. 
Thofe who are defirous of peruflng a fuller account of 
the hiftory of the progrefs of navigation may confuit Dr. 
James Wilfon’s preface to Dr. Robertfon’s Elements of 
Navigation, in two volumes, 8vo. 177a ; a work defervedly 
held in the higheft eftimation. The only works that have 
appeared of late on the fubjeCt are thofe on Longitude 
and Navigation by Dr. Andrew Mackay, of Edinburgh, 
in 1803 ; works of high reputation, and of which we have 
availed ourfelves in the following part of this article ; and 
M. de Roffel’s Practical Treatife on finding the Latitude 
and Longitude at Sea ; tranllated from the French by Mr. 
Myers, of the Academy at Woolwich, 1817. 
THEORY and PRACTICE. 
It may undoubtedly be faid, that in no country in the 
world is a correCt knowledge of navigation fo highly im¬ 
portant as our own, on account of the magnitude of our 
navy, the extent of our commerce, and the number of ouf 
foreign poffeflions. 
The foundations of the principles of this art are ex¬ 
plained in other parts of our work, under the articles 
Astronomy, Geography, Horology, Log, Loga¬ 
rithms, Compass, &c. What we (hall therefore find it 
neeeffary here to do, is to apply what has been already 
delivered of the theory to the praCtice under our imme¬ 
diate confideration; digefting the whole in a methodical 
manner. 
The motion of a Ihip in the water is well known to de¬ 
pend on the aCtion of the wind upon its fails, regulated 
by the direction of the helm. As the water is a refilling 
medium, and the bulk of the Ihip very confiderable, it 
thence follows that there is always a great refiftance on 
her fore-part; and, when this refiftance becomes fuflicient 
to balance the moving-force of the wind upon the fails, 
the Ihip attains her utmoft degree of velocity, and her 
motion is no longer accelerated. This velocity is different 
according to the different ftrength of the wind ; but the 
ftronger the wind, the greater refiftance is made to the 
Clip’s paffage through the water : and hence, though the 
wind fhould blow ever fo ltrong, there is alfo a limit to 
the velocity of the fliip : for the fails and ropes can bear 
but a certain force of air; and, when the refiftance on the 
fore-part becomes more than equivalent to their ftrength, 
the velocity can be no longer increafed, and the rigging 
gives way. 
The direction of a Ihip’s motion depends on the pofition 
of her fails with regard to the wind, combined with the 
aCtion of the rudder. The molt natural direction of the 
Clip is when fhe runs directly before the wind ; the fails 
are then difpofed fo as to be at right angles thereto. But 
this is not always the cafe, both on account of the vari¬ 
able nature of the winds, and thefituation of the intended 
port, or of intermediate headlands or iflands. When the 
wind, therefore, happens not to be favourable, the fails 
are placed fo as to make an oblique angle both with the 
direction of the Ihip and with the wind ; and the fails, 
together with the rudder, muftbe managed in fuch a man¬ 
ner, that the direction of the ihip may make an acute 
angle with that of the wind ; and the ihip, making boards 
on different tacks, will by this means arrive at the in¬ 
tended port. 
The reafon of the ihip’s motion in this cafe is, that the 
water refills the fide more than the fore-part, and that in 
the fame proportion as her length exceeds her breadth. 
This proportion is fo confiderabie, that the ihip continu¬ 
ally flies oft' where the reliltance is ieait, and that fome- 
times with great fwiftneis. In this way of failing, how¬ 
ever, there is a great limitation : for, it the angle made 
by the keel with the direction ot the wind be too acute, 
the ihip cannot be kept in that polition 5 neither is it 
poflible for a large ihip to make a more acute angle 
with the wind than about fix points; though fmall Hoops, 
it 
