V.—Scientific Intelligence.
I.—Progress of the Indian Trigonometrical Survey.
Plate I.
The measurement of the base line on the Barrackpúr road was completed on the 21st January, and it is difficult to imagine that any similar work was ever brought to a more successful issue. Through the politeness of Captain Everest, the Surveyor General and Superintendent of the Trigonometrical Survey, we enjoyed the advantage of an invitation to witness the remeasurement of the first day's work, with the view of ascertaining what might be the probable amount of error: on which occasion the President of the Physical Class of the Asiatic Society and many distinguished officers of the Engineer department were present. An elegant breakfast was laid out in tents after the ceremonies of the morning were concluded. While contemplating with admiration the order and precision with which the whole process was conducted, we took an opportunity of sketching the apparatus as it stood, that the readers of the Journal might be better able to comprehend the nature of the operation of measurement, which was partially described in the Gleanings for November last.
Plate I. represents the six sets of bars resting upon their tripods, levelled, and in the act of adjustment, longitudinally, by means of the directing or boning telescope, to the left hand. The boning telescope ought to have been considerably more distant from the bars, but it would then necessarily have been excluded from the drawing. A movable covering of tent-frame work protects the bars from the influence of the morning sun; at their left extremity is seen a cast-iron tripod, firmly imbedded in the ground, bearing a brass vertical cylinder and plate, upon the surface of which is the minute dot which marks the termination of the last, and acts as the starting point of the present measurement: by the adjustment of the cross wires of the end microscope in the true vertical line bisecting the dot. These apparatus are represented on a larger scale in the foreground; as also one of the wooden boxes containing the compound bar, shewing the two projecting tubes, within which lie the cross levers of the compound bars, upon which are engraved the dots, or marks to be read off by the double microscopes interposed between each box, as described on a former occasion, and as will be readily comprehended by reference to the drawing. The right extremity of the line is seen to enter the door of the tower, where it terminated in a coincidence with the original dot, engraved upon a metallic disc attached to a sunken stone pier.
As it had been anticipated that the settling of the masonry of the tower might derange the terminal mark, precautions had been taken in the first instance to sink into the ground another adjusting point at a short distance in advance of the tower; and it was in fact to this point that the remeasurement was referred, to know the probable amount of error in measuring, as well as whether the tower had altered its position in any perceptible degree.
From conversation with the officers on the ground we picked up the following particulars regarding the Barrackpur base. We must crave their indulgence if our memory leads us into any mistake in detailing them.
The measurement commenced on the 23rd November 1831, and ended on the 21st January 1832, an interval of 58 days, of which 13 may be set down as holidays; so that the actual time employed was about 45 days. The length of the ground measured upon an average was 750 feet, or 12 sets of bars: but towards the conclusion, so systematic had become the arrangements, that 18, 20, and once 24 sets (that is 1512 feet) were measured in one day, which is double what was effected on
