Page:Journal of the Asiatic Society of Bengal Vol 1.djvu/374

332 In 1823, Captain Ross was appointed by the Court of Directors to his present office of Marine Surveyor General. His operations were necessarily interrupted during the Burmese war ; but the following surveys have since been executed in succession under his superin- tendence, and mostly by himself personally.

1. The Rangoon river to its mouth.

2. The straits, approaches, and harbour of Singapore.

3. The Mergui archipelago, Tenasserim coast, and Martaban, with the river at Amherst and Moulmein. The sheets of this Chart cover a line of coast extending fromLat. 8°. 28'. N. to 16°. 32'. and include a vast number of Islands never before laid down or even visited by Europeans,

4. In the meantime, the coast of Ava from Negrais to Ramri, and Sandowi, was surveyed by Captain Crawfurd, and Cheduba roads and Ramri, by Captain Ross's assistant Lieut. Lloyd.

5. The coast of Arracan, north of the point to which Captain Crawfurd's Chart extended, was, in the past season, the object of Captain Ross's personal survey ; and, in one or two seasons more, the entire eastern shore of the Bay of Bengal will have been laid down by this officer, or by those under his orders, with as much accuracy as can be claimed for the charts of the coasts of Europe and the Mediterranean.

All Captain Ross's surveys, and those made under his orders by junior officers of the department, are laid down from bases carefully measured on shore, where this has been possible, and are strictly tri- gonometrical ; and though necessarily wanting that minute correctness aimed at in similar surveys on land, they possess, nevertheless, an accuracy fully sufficient for the scale on which the Charts are delineated. The base lines on shore are measured on the most favorable level spots that can be found, by running a ten-foot rod along a cord, stretched tight between the extreme points, and kept in position by stakes, of which the direction is verified by a telescope at one end. Second and third bases are measured for further assurance, and in correction of the first.

If there be no means of measuring a base on shore, as when the locality of rocks and sand-banks, out of sight of land, may have to be ascertained, recourse is had to the measurement of a base by sound, which in a long line of 5^ to 6 geometric miles is a process affording more practical accuracy than would be supposed. The vessels being an- chored at this distance, and a calm period chosen, the distance is taken between the flash and report of a gun, and upon the assumption that sound travels at the rate of 1140 feet per second, while with repeated

Note.—The Andamau Islands were surveyed as far back as 1789, 90, and 1793, by Captain Blair.