"تاريخ البحرية الهندية. (١٦١٣- ١٨٦٣)." [‎٤‎٠‎٥] (٦٢٢/٤٢٤)

HISTORY OF THE INDIAN NAVT.
405
plished. On the conclusion of the war, Lieutenant Constable
was ordered to correct* the survey of the Gulf, which occu-
* Lieutenant Constable's instructions were briefly comprised in the following
^aph of a despatch, dated the 24th of June, 1857, from the Court of
directors to the Grovernment of Bombay. " It is very important that as soon as
the demands of the Service will admit of it, one of the Indian Navy vessels should
be employed in making a circuit of the Persian Gulf, for the purpose of determin
ing correctly the latitudes and longitudes of the principal points." " In accord
ance with these instructions," says Captain Constable, in an official memorandnni
of his proceedings, " certain positions round the Q-ulf were carefully determined,
afc such short distances apart as would admit of the interjacent coast-line being
completed from the existing charts without material error, a vessel being thereby
enabled to shape her course from one point to another with certainty. The new
chart thus constructed, is a correct skeleton of the islands, and of the principal
points and towns around the Q-ulf, with as much of the detail of coast-line (in
the vicinity of the positions fixed) correctly triangulated as the time admitted,
the remainder of the coast-line being adapted from the old charts. The latitudes
and longitudes were mostly astronomically determined by reflecting instruments of
the best class, and with the exception of a few observations, at shoals not dry,
out of sight of land, they were exclusively made with the artificial horizon. The
latitudes were determined by observations of the pole star, and by circum-
meridional altitudes of stars, and, when the altitude was not too great, of the
sun. Care was taken to obviate any errors in the usual tables of refraction, by
observing stars on both sides of the zenith, and, to compensate possible errors of
construction, by using at the most important stations, three different instruments,
viz., a reflecting circle, and two sextants. With the exception of some minor
points, the observations at each station were numerous and varied. The details
of these observations, as well as those for meridian distances, were given in a
register, which was forwarded with the chart to G-overnment. The longitudes
were determined chronometrically by means of seven chronometers. The obser
vations for time were altitudes of the sun all taken before noon, as near the same
time and under the same circumstances as practicable; and also, by the same
observer, with the same instrument, viz., a Troughton's reflecting circle, so that
inaccuracies arising from index error, inaccurate graduation, &c., were practically
compensated. The general system pursued was to start from some well-deter
mined rating station, taking observations at several, not too distant, circum
jacent points, and return, without delay, to the starting-point, to obtain a mean
rate for the interval, which was not to exceed ten or twelve days, the rating
stations being connected with the fundamental positions by at least two in
dependent measurements. It was found possible, gradual!v, during the course
of the chronometric observations between Kais (Kenn), Kishm, and Limeh, to
connect these and the intermediate stations, also trigonometrically, thereby
checking the observations. Numerous minor points, and all the principal hills
and land-marks, were fixed trigonometrically from the astronomical positions.
The variation of the compass, which is westerly, and had much decreased since
the former survey, was everywhere carefully determined by azimiths observed on
shore with a 7-inch Everest theodolite. No information being given on the old
charts as the heights of mountains, islands, &c., much attention was bestowed
on their accurate determination, elevations being observed by the 7-inch theodo
lite, and heights above mean level of the sea computed. During the surveying
vessel's progress from one station to another, a great many soundings were taken
to fill up gaps, and also test those exhibited on the old charts. The nature of the
bottom was likewise everywhere noted, as it had been rarely recorded by former
surveyors."
In a letter addressed to Commodore Jenkins, commanding the Persian Gulf
Squadron, dated the 27th of April, 1858, Lieutenant Constable gives the follow
ing details of the work up to that date :—
" I arrived at Bushire from Bombay on the 25th of December, 1856, and was
immediately sent up a narrow creek, where the vessel was stationed two miles
inland of Bushire, as a floating battery for the protection of the lelt flank of the
British camp, and was detained there until the 25th of April following. During