1830 ] 
On the Tides of the River Hi<gli. 
33 
observations ; because the design has been to afford a practical inference of the 
degree of accuracy which an observer may expect with the means with which he is 
furnished on the spot. It may be proper to mention also, that the table includes 
every set of distances observed at the stations to which it refers. 
The conveniency of the circle in observation, and the facility with which it may 
he managed by those who will accustom themselves to its use, may be judged by 
the observations at Jamaica ; where it may be seen that 60 distances of the Sun and 
Moon were observed within the hour, or one in each minute, including the obser- 
vation and entry of the time of each distance, and the reading off at every tenth dis- 
tance, and writing down the arc passed through. In the repetition of the same 
process with the Moon and Aldebaran at night, the number of distances observed in 
an hour was 50, or, on an average, one in a minute and twelve seconds. Of the 
six sets, into which the distances of the Sun and Moon under notice were divided, 
three are within one mile of the combined result of twenty-seven sets, and the two 
others within two miles ; and of the five sets of the Moon and Aldebaran, four are 
within one mile, and the fifth within two miles : this is stated to shew that accu- 
racy was not sacrificed to expedition ; both the instances were without the advan- 
tage (in expedition certainly) of an assistant. 
The observation of the angular distance of the Moon, from certain fixed stars, 
has long and universally been regarded as the best means of deducing the longitude 
of a vessel on the ocean from celestial phenomena ; but it has not been so generally 
recognised as it deserves to be, as the most eligible of all the methods which pre- 
sent themselves to the choice of the geographer, or the practical astronomer, for 
determining positions on land, wherever time or the conveyance of instruments 
form a part of the consideration. It combines, in a degree far beyond comparison 
with any other method, the very important qualities of convenience, expedition, 
and accuracy. The whole apparatus which is required, — a circle, a chronometer, 
and an artificial horizon, — does not weigh twelve pounds; no temporary observa- 
tory is required for its protection, and all situations are equally convenient for its 
use ; the latitude and longitude may both be determined in the first 24 hours after 
the arrival at a station, during three-fourths of every lunation ; and as the observa- 
tions by which the determinations are accomplished, may be multiplied within that 
interval at the pleasure of the observer, so as to comprise, in respect to latitude, 
every important variety of circumstance, and almost every variety in regard to 
longitude, no sacrifice of accuracy to expedition is called for, hut the precision will 
be proportionate to the labour which is bestowed. 
There are occasions in which the qualities of convenience in portability, expedition, 
and accuracy in determination, are almost equally essential. Such is the design 
which is understood to be entertained, of forming the bases of a survej' of central 
India, by the celestial determination of the geographical position of stations, select- 
ed at proper intervals, over that very extensive portion of the globe. Admitting 
the space to contain 130 or 140 square equatorial degrees, and the stations to be, 
on an average, 100 miles apart, above eighty such determinations must he made. 
Those who are acquainted with the apparatus which would he required, in any other 
mode of deducing the position from celestial observation than the one under notice, 
and will pursue, in detail, the consideration of the conveyance of such an apparatus 
over such an extent of country, independently of the accidents and interruptions 
to which it would be liable, — and who can appreciate the time which would be 
occupied in obtaining an equally precise determination at each station, as lunar 
distances would give in 24 hours, — will, I think, arrive at the conclusion, that it 
is only by lunar distances that the design is likely to receive its accomplishment. 
HI, — On the Tides of the River Hugli. 
In a former paper on the subject of tides, which appeared in your eleventh num- 
ber, while drawing a comparison between the tides observed at Calcutta, and those 
observed at the mouth of the river in the island of S&gar, I was led into a train 
of conclusions upon the general principles affecting the level of the tides in then 
progress inland, which may, perhaps, appeal more forcibly to the understandings of 
