The stations chosen for the survey of Essex, and parts of the adjoining counties, as also for completing the survey of Kent, are mostly towers, as may be seen from the above. When the tops of the towers have been smooth and even, the stations were always in the centres of them; but, when they were covered with roofs, or had spires upon them, stations were chosen in the most convenient places, and staffs always erected. I have omitted giving the measurements by which the stations may be exactly found, Rayleigh and Prittlewell excepted, in order to avoid swelling this article to an inconvenient length. ART. VIII. Particulars relating to the Base on King's Sedgemoor, and the Reduction of that Base. Plate XXVIII. Comparisons of the Chains. As the chains, after the measurement on Salisbury Plain, were oiled, and laid up in the Tower, no apprehensions were entertained that either of them was elongated by the rusting of the joints. It was, however, our wish to have compared them with each other, previous to the commencement of this operation, and attempts were made, but rendered unsatisfactory, from the want of sufficient firmness in the soil. It was not till we arrived at the 70th chain, that a good opportunity presented itself: the measuring chain A, was then compared with the standard B, and found to be thirteen divisions of the micrometer head, attached to the brass scale, in excess. In these trials, the temperature remained constant; the mercury in FAHRENHEIT'S thermometer being at 66°. The 50-feet chain, spoken of in a former article, came from the hands of Mr. RAMSDEN without being very accurately measured; therefore it now became proper to ascertain its length, by means of the standard chain. This was accordingly done at the present time; when B was found to exceed twice the length of the 50-feet chain, by 14 divisions of the micrometer screw; the thermometer, at the time of trial, standing at 6910. At the conclusion of the measurement, the chains were again. compared, when the working chain A, was found to exceed the standard, 174 divisions on the micrometer head this was after 273 chains were measured. Now, when 70 chains only had been measured, the difference between A and B was 13 of those divisions; consequently 174-13, 44 divisions, was the wear of B, in measuring 203 chains. Therefore, the whole wear is found by this proportion, viz. 203: 44:: 273: 5,223 divisions, =1 of an inch; which very inconsiderable quantity, like the wear on Salisbury Plain, no doubt, arose from the pivots and pivot holes of the joints being polished by continual use. This supposition seems just; as the wear of the chain, after the measurement on Hounslow Heath, was found to be much greater. The length of the chain A, as well as that of the standard B, was accurately ascertained by Mr. RAMSDEN, in the year 1793, as particularly shewn in the Philosophical Transactions for 1795. In the temperature of 54°, A was found to exceed 100 feet, 11435 of an inch; therefore, adding the wear which took place on Salisbury Plain, viz. part of an inch, we get the length of A at the commencement of the measurement on Sedgemoor 100,01009 feet. 100 I From repeated trials, as before observed, the standard B was found to exceed the length of twice that of the new fifty-feet chain, 14 divisions of the micrometer head; and, after the measurement, the same chain fell short of A, 174 of those divisions: hence, A exceeds twice the length of the 50-feet chain, 314 divisions. Therefore the length of the short chain, in the temperature of 54°, may be taken at 50,00075 feet. ART. X. Reduction of the Base. The overplus of the 273d chain was measured by Mr. RAMSDEN, and found to be 23,517 feet; wherefore, the apparent length of the base was From the measurement in the Riding-house of his Grace the Duke of MARLBOROUGH, the chain A was found to exceed 100 feet, in the temperature of 54o, 0,11425 parts of an inch; to which, adding the wear by the measurement on Salisbury Plain, viz. z。, and also half the wear by the measurement of this base, viz. part of an inch, 0,1191 100 we get for the excess of the chain's length feet; which add 12 The sum of all the degrees shewn by the thermometer was 98511; wherefore,98511 — 54° × 272,8 x 0.0075=3,1069 feet; which also add 12 Again, from the comparison of the 50-feet chain with the standard B, it appeared that the excess above 50 feet, in the temperature of 54°, was 0,09075 parts of an inch; therefore, 0,09075 × 8=0,0605 12 parts of a foot. This likewise add The sum of all the degrees shewn by the thermometers placed by the sides of the 50-feet chain, Feet. 27676,4830 +2,7075 +3,1069 +0,0605 1372 - 54° × 4× was 1372; therefore parts of a foot: and this add ,0075=0,0365 12 5 +0,0365 27682,3944 |