G28 
NAVI G 
Of TAKING OBSERVATIONS at SEA. 
It is well known, that, although the courfe and diftance 
between any two places, whofe latitude and longitude 
are accurately fettled, may be afcertained with the greateil 
precifion, yet a (hip at lea is fo liable to be put out of the 
proper courfe, by ftorms, contrary winds, currents, &c. 
that, after all reafonable allowance is made for the errors 
arifing from thel'e caufes, the place of the (hip by account 
is very feidom found to agree with its real place, or that 
deduced from obfervation, the difference frequently 
amounting to feveral degrees. It is therefore obvious 
that the (hip may be expofed to the greateft hazard when 
the feamen think themfelves moil fecure; and from this 
it appears that there is an abfolute necellity of taking ob- 
J'ervations, whenever an ooportunity offers, for obtaining, 
with accuracy, the true place of the flip. 
At fea, the latitude of a (hip is eafily deduced either 
from the meridian altitude of the fun, or from that of a 
liar or planet, or from double, treble, &c. altitudes of any 
of thefe objects; but, with regard to the longitude, a 
more difficult operation becomes necelfary. At land, 
indeed, the longitude of any place may be found with 
very little trouble by feveral methods, particularly by the 
eclipfes of Jupiter’s fatellites ; and the longitude at fea 
might be found by this method with the fame facility, 
provided a telefcope polfeffed of a fufficient magnifying 
power could be employed for that purpofe. But this is 
found to be extremely difficult, by reafon of the perpetual 
agitation of the fea. However, attempts to render a 
teiefcope manageable at fea have been made by Dr. Irwin 
and others; and an inftrument for this purpofe is de- 
fcribed in Rutledge’s Theorie Altronomique. But a tele- 
fcone with a fmall magnifying power may be ufed at fea in 
moderate weather; and, fince, even with fuch an inftru¬ 
ment, folar eclipfes and occultations may often be ob- 
ferved with tolerable accuracy, and confequently the 
longitude may be afcertained from fuch obfervations, the 
opportunities of making them ought never to be negledled 
when the ffiip is either out of fight of land, or near an 
unknown coaft. The calculations, indeed, which necef- 
farily attend thefe methods, feem to be confidered as too 
laborious and difficult for the generality of feamen ; and, 
therefore, the method by obferving the diftance between 
the moon and the fun, or a fixed liar, is now commonly 
preferred, both on account of its l'uperior facility, and 
becaufe it can be praftifed at fea ahnoft as often as necef- 
fary. Yet, as even this method is attended with a calcu¬ 
lation that is, by many navigators, thought troublefome, 
various ways have been propofed to ffiorten, as much as 
pollible, the operations for reducing the apparent to the 
true diftance; and it is now accordingly reduced to a 
tolerably fimple computation. By this method the lon¬ 
gitude may, in molt cafes, be determined within half a 
degree, which at fea is efteemed no very great error; 
and, if the calculations are accurately performed, from 
the mean of feveral fets of good obfervations, taken at 
fhort intervals of time, the error will probably be greatly 
diminilhed. 
If a chronometer or time-keeper could be conftrufled, 
fo as to go uniformly when placed in every different po- 
fition, and under different degrees of heat, then would 
this method of finding the longitude at fea be a molt va¬ 
luable acquifition to the navigator. Indeed fome of our 
ingenious countrymen have brought this art to a degree 
of perfection formerly unknown ; and every perfon ac¬ 
quainted with the principles of watch-making mull highly 
admire the accuracy of Harrifon’s, time-keeper. Thole 
made by Melfrs. Arnold, Kendal, Earnlhaw, Barraud, 
See. are alfo excellent; but it is to be hoped that inftru- 
ments of this kind may be Hill farther improved, and may 
be afforded much cheaper than at prefent; for the high 
price alone is a very great objection to them, and very 
much prevents their being more generally ufed. It is 
well known that every chronometer hitherto contrived 
4 
A T I O N. 
is fubjeCt to irregularities; the finalleft (hock is found 
affeCt them, and the rate of going is found to be altered 
by changes in the atmofphere,even though provided with 
thermometer-pieces. Upon this and other accounts, 
therefore, their accuracy is very much to be fufpeCted ; 
fo that at prefent they are chiefly ufed for experiments, 
or to conned obfervations, for which purpofe they cer¬ 
tainly make a valuable appendage to a fet of nautical 
inftruments. See a defeription of the moll approved 
longitude time-pieces under the article Horology, vol. 
x. p. 287-36+. and direClions for the ufe of them, 
p. 364.-367. 
To find the Apparent Time, and thereby regulate the 
going of the Watch. 
It is necelfary here to premife, that there are three di- 
vifions of time in ufe; the civil, the altronomical, and the 
nautical. 
The civil day begins at midnight, and ends at the mid¬ 
night following, being divided into two equal parts of 
twelve hours each ; the firft twelve being marked A. M. 
that is, ante meridiem, or before noon; the latter twelve 
P.M. that is, pojl meridiem, or afternoon. This diviiion. 
of time is moll generally ufed. 
The aftronomical day, fo called from its being ufed by 
aftronomers, begins at the noon of the civil day, and con¬ 
tinues to the noon of the civil day following i the hours 
being counted in regular fucceffion from one to twenty- 
four), fo that the firft part of the aftronomical day is the 
laft part of the civil day : aqd the lad part of the allro- 
nomical day includes the firft part of the civil day fol¬ 
lowing. 
The nautical day, in ufe amongll feamen,'is, in one 
refpedt, the direft reverfe of the aftronomical day, as it 
ends when the aftronomical day begins. This it has in 
common with the civil day, that it is divided into two 
equal parts of twelve hours each, but the firft twelve hours 
are marked P. M. and the latter twelve A. M. An ex¬ 
ample will bell illuftrate this. By the lea-reckoning, 
Tuefday begins immediately after meridian on Monday 5 
all occurrences happening from Monday noon to mid¬ 
night, though the firft part of Tuefday by the nautical 
reckoning, are marked as happening at fuch an hour 
P.M. and all occurrences happening from midnight to 
Tuefday noon are marked as happening at fuch an hour 
A. M. Thus it appears that the hours in the nautical 
day are regulated by the civil day, but the nautical day 
itlelf begins twelve hours before the civil day. From, 
what h3s been faid, it will appear that the noon of the 
civil day, the beginning of the aftronomical day, and 
the end of the nautical day, take place at the fame time. 
The different kinds of time are two, mean and appa¬ 
rent Mean time is that ffiown by a clock or watch regu¬ 
lated to mean folar time. Apparent time is reckoned from 
the paffage of the fun over the meridian of any place. 
Mean and apparent time will fometimes differ from each, 
other near a quarter of an hour, owing to the irregularity 
of the earth in her orbit, or the variation in the inclina¬ 
tion of her axis. This difference is called the equation 
of time, and is contained in page 2 in the Nautical Al¬ 
manac. It is only requifite to take notice of it in deter¬ 
mining the longitude by a time-keeper, but not in any 
other nautical oofervation, as the calculations in the 
Nautical Almanac are adapted to apparent time. 
To find the Apparent Time by equal Altitudes of the Sun. 
Take the fun’s altitude at any convenient time in the 
forenoon, two, three, four, or five, hours diftant from the 
meridian ; fet down the altitude with correfponding time 
by watch exaftly; fet the index to the fame altitude, and 
wait till the fun comes to that altitude in the afternoon 5 
note the time by watch ; half the fum of thefe two times 
is the apparent time Ihown by the clock or watch, when 
the fun was on the meridian of that place. But it mull 
hers: 
