VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 635 



Beacon, Wiltshire: this hill is above the village of Inkpin, and the station is in 

 the centre of the small field circumscribed by a ditch and parapet of an ancient 

 fortification. — Highclere, Wiltshire: the station is in the centre of the Ring on 

 Beacon Hill, about half a mile south-east of Highclere. — Bagshot Heath: the 

 station is on the brow of an eminence 2 miles north of the Golden Farmer, and 

 directly west of the north corner of Bagshot Park. — Hind Head, Surrey: the 

 station is near the gibbet, being about 12 feet north-west of it. 



As it is probable that some individual will avail himself of the particulars 

 given in this performance, by forming more correct maps of the counties over 

 which the triangles have been carried, and who consequently may wish to visit 

 certain of the stations, it is proper to observe, that small stakes are placed over 

 the stones sunk in the ground, having their tops projecting a little above it. 

 For some years there will be little difficulty in finding the stations, as the spots 

 are well known to the neighbouring inhabitants. 



Next follows the measurement of the base of verification on Salisbury Plain 

 with the hundred feet steel chain, in the summer of the year 1704. 



The apparatus with which this base was measured arrived at Beacon Hill the 

 25th of June, and consisted of the 2 steel chains, the tressels belonging to the 

 R. s., and the 20 coffers used on Hounslow Heath, with the pickets, iron-heads, 

 and a few other articles, which in the beginning of this year had been made at 

 the Tower. As it was foreseen that the truth of this measurement would, in a 

 great degree, depend on the accurate reduction of the several hypotenuses to the 

 plane of the horizon, an application was made to Mr. Ramsden in the foregoing 

 winter, to consider of some means by which their inclinations might be ob- 

 tained. He therefore applied an arch to the side of the transit telescope, which 

 he divided into half degrees; and opposite to this he placed a microscope, with 

 a moveable wire in its focus, by means of which, and the micrometer of the 

 telescope, an angle could be taken. 



On the first convenient opportunity after the arrival of the apparatus, we 

 determined the value of any number of revolutions of the micrometer-screw in 

 parts of a degree, by the following method. At the distance of 100 feet from 

 the transit, a picket was set up, on which a dot was made with chalk, and the 

 instrument being adjusted, was moved by the finger-screw till the edge of the 

 micrometer-wire touched some prominent part of that mark. The wire in the 

 focus of the microscope was then made to bisect a dot on the arch, and the 

 telescope moved in the vertical till the next dot was bisected, by which the in- 

 strument had described half a degree on its axis, and the micrometer-wire was 

 afterwards moved till it touched the same part of the chalk mark, the revolutions 

 being counted, which were consequently equal to 30'. This operation was re- 

 peatedly tried, with a picket placed from 1 to 6oo feet successively from the 



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