duced a timekeeper of his o'.vn construc- 
tion, which did not err above one second in a 
month for ton years together; and in the year 
17 36 he liad a machine tried in a voyage to 
and from Eisbftn, which was the means of 
correcting an error of almost a degree and a 
hall in the .computation of the ship’s reckon- 
ing. In consequence of this success, Mr. 
Ilamson received public encouragement to 
roceed, and he made three other time- 
ee pers each more accurate than the for- 
mer, which w^ere finished successively in the 
years 1739, 1 75S, and 1761 ; the last of 
which proved so much to his own satisfac- 
tion, that he applied to the commissioners of 
the longitude to have this instrument tried in 
a voyage to some port in the West Indies, 
according to the directions of the statute of 
the 12th of^Anne above cited. Accordingly, 
Mr. William Har rison, son of the inventor, 
embarked in November 1761, on a voyage 
for Jamaica, with this fourth timekeeper or 
watch; and on his arrival there, the longi- 
tude, as shewn by the timekeeper, differed 
but one geographical mile and a quarter from 
the true longitude, deduced from astrono- 
mical observations. The same gentleman 
returned to England with the timekeeper, in 
March 1762, when he found that it had erred 
in the four months, no more than V 54'k§ in 
time, or 2 8f minutes of longitude; whereas 
the act requires no greater exactness than 30 
geographical miles, or minutes of a great 
circle, in such a voyage. Mr. Harrison now 
claimed the whole reward ©f 20,000/. offered 
by the said act: but some doubts arising in 
the minds ol the commissioners concerning 
, the true situation of the island of Jamaica, 
| and the manner in which the time at that 
place had been found, as well as at Ports- 
I mouth ; and it being farther suggested by 
j some, that although the timekeeper luip'- 
pened to be right at Jamaica, and after its 
| return to England, it was by no means a 
proof that it had been always so in the inter- 
I mediate time; another trial was therefore 
I proposed, in a voyage to the island of Bar- 
| badoes, in which precautions were taken to 
I obviate as many ot these objections as pos- 
I sible. Accordingly the commissioners pre- 
I viously sent out proper persons to make 
J astronomical observations on that island, 
which, when compared with other corre- 
; sponding ones made in England, would de- 
termine, beyond a doubt, its true situation; 
and Mr. William Harrison again set out with 
his father’s timekeeper, in March 1764, the 
watch having been compared with equal al- 
titudes at Portsmouth before he set out, and 
he arrived at Barbadoes about the middle of 
May; where, on comparing it again by equal 
! altitudes of the sun, it was found to shew the 
: difference of longitude between Portsmouth 
and Barbadoes to be 3h. 55m. 3s.; the true 
difference of longitude between these places, 
by astronomical observations, being 3h. 54m! 
20s.; so that the error of the watch was 43s., 
or 10' 45" of longitude. In consequence of 
this and the former trials, Mr. Harrison re- 
| ceived one moiety of the reward offered by 
the 12th of queen Anne, after explaining the 
principles on which his watch was construct- 
i ed, and delivering this, as well as the three 
former, to the commissioners of the longi- 
tude for the use of the public: and he was 
; promised the other moiety of the reward, 
when other timekeepers should be made on 
LONGITUDE. 
the same principles, either by h'msclf or 
others, performing equally well with that 
which he hati last made, in tire mean time, 
this last timekeeper was sent down to the 
Royal Observatory at Greenwich, to hatred 
there under the direction of the Rev. Dr. 
Maskelyne, the astronomer-royal. Put it 
did not appear, during this trial, that the 
watch went with the regularity that was ex- 
pected ; from which it was apprehended that 
the performance even of the same watch was 
not at all times equal ; and consequently that 
little certainty could be expected in the per- 
formance of different ones. Moreover the 
watch was now found to go faster than dur- 
ing the voyage to and from Barbadoes by 18 
or 19 seconds in 24 hours; but this circum- 
stance was accounted for by Mr. Harrison, 
who informs us that be had altered the rate 
ot its going by trying some experiments, 
which h.e had not time "to. finish before he was 
ordered 10 deliver up the watch to the board. 
Soon after this trial, the commissioners of 
longitude agreed with Mr. Kendal, one of 
the watch-makers appointed by them to re- 
ceive Mr. Harrison’s discoveries, to make 
another watch on the same construction with 
this, to determine whether such watches 
could be made from the account which Mr. 
Harrison had given, by other persons as well 
as himself. The event proved the affirma- 
tive; for the watch produced by Mr. Kendal, 
in consequence of this agreement, went con- 
siderably better than Mr. Harrison’s did. 
Mr. Kendal’s watch was sent out with captain 
Cook, in his second voyage towards the south 
pole and round the globe, in the years 1772, 
1773, 1774, and 1775; when the only fault 
found in the watch was, that its rate of going 
was continually accelerated; though in this 
trial of three years and a half it never 
amounted to 1 4C| a day. The consequence 
was, that the house of commons, in 1774, to 
whom an appeal had been made, were pleas- 
ed to order the second moiety of the reward 
to be given to Mr. Harrison, and to pass the 
act above-mentioned. Mr. Harrison had also 
at different times received some other sums 
of money, as encouragements to him to con- 
tinue his endeavours, from the board of lon- 
gitude, and from the India company, as well 
as from many individuals. Mr! Arnold and 
some other persons have since also made se- 
veral very good watches for the same pur- 
pose, and have been remunerated for their 
skill and labour. 
Others have proposed various astronomical 
methods for finding the longitude. These 
methods chiefly depend on having an ephe- 
meris or almanac suited to the meridian of 
some place, as Greenwich for instance, to 
which the Nautical Almanac is adapted, 
which shall contain for every day computa- 
tions of the times of all remarkable celestial 
motions and appearances, as adapted to that 
meridian. So that if the hour and minute is 
known when any of the same phenomena are 
observed in any other place whose longitude 
is desired, the difference between this time 
and that to which the time of the said phe- 
nomenon was calculated and set down in the 
almanac, will be known, and consequently 
the difference of longitude also becomes 
known between that place and Greenwich, 
allowing at the rate oi lifteen degrees to an 
hour. 
Now it is easy to find the time at any 
6 b 
place, by means of the altitude or azimuth 
of the sun or stars, which time it :s w cessary 
to find by such means, both in these astrono- 
mical modes of determining the longitude, 
and in the former by a timekeeper; and it 
is the difference between that time, so de- 
termined, and the time at Greenwich, known 
either by the timekeeper or by the astrono- 
mical observations ot celestial phenomena, 
which gives the difference of longitude at the 
rate above-mentioned. Now the difficulty 
in these methods lies in the fewness ol proper 
phenomena, capable of being thus observed ; 
tor all slow motions, such as belong to the 
planet Saturn, for instance, are quite exclud- 
ed, as affording too small a difference, in a 
considerable space of time, to be properly 
observed; and it appears that there are no 
phenomena in the heavens proper for this 
purpose, except the eclipses or motions of 
Jupiter’s satellites, and the eclipses or mo- 
tions of the moon, viz. such as her distance 
from the sun or certain fixed stars lying near 
her path, or her longitude or place in the zo- 
diac, &c. Now of these methods, 
1st. That by the eclipses of the moon is 
very easy, and sufficiently accurate, if they 
did but happen often, as every night. For 
at the moment when the beginning or middle 
or end of an eclipse is observed by a tele- 
scope, there is no more to be done but to 
determine the time by observing the altitude 
or azimuth of some known star; which time 
being compared with that in the tables, set 
down for the happening of the same pheno- 
menon at Greenwich, gives the difference in 
time, and consequently of longitude sought. 
But as the beginning or end of an eclipse of 
the moon cannot generally be observed nearer 
than one minute, and sometimes two or three 
minutes of time, the longitude cannot cer- 
tainly be determined by this method, from a 
single observation, nearer than one degree of 
longitude. However, by two or more obser- 
vations, as of the beginning and. end, & c. a 
much greater degree of exactness may be 
attained. 
,2d. The moon’s place in the zodiac is a- 
phenomenon more frequent than her eclipses; 
but then the observation of it is difficult, and 
the calculus perplexed and intricate, by rea- 
son of two parallaxes; so that it is hardly 
practicable to any tolerable degree of accu- 
racy. 
3d. But the moon’s distances from the sun 
or certain fixed stars, are phenomena- to be 
observed many times in almost every night, 
and afford a good practical method of deter- 
mining the longitude of a ship at almost any 
time; either by computing from thence the 
moon’s true place, to compare with the same 
in the almanac; or by comparing her ob- 
served distance itself with the same as there 
set down. 
From the great improvements made by 
Newton in the theory of the moon, and more 
lately by Euler and others on his principles, 
professor Mayer, of Gottingen, w 7 as enabled 
to calculate lunar tables more correct than 
any former ones ; having so far succeeded as 
to give the moon’s place within one minute 
of the truth, as has been proved by a compa- 
rison of the tables with the observations 
made at the Greenwich observatory by Dr. 
Bradley, and by Dr. Maskelyne, the late 
astronomer-royal ; and the same have been 
