636 PHILOSOPHICAL TRANSACTIONS. [aNNO 1795. 



telescope, the runs of the micrometer-screw being in each case nearly the same, 

 as indeed they ought to be according to theory. The number of revolutions 

 equal to 30' was found, fi'om a mean of these trials, to be 12-j-L-. Having de- 

 termined this, the chains a and b were compared with each other, when they 

 were found to have the same difference of lengths as when measured by 

 Mr. Ramsden. 



The experience obtained in the measurement of the base on Hounslow Heath, 

 led us to discover, that some of the methods to execute particular parts of it, 

 might have been improved. One of them was, the means by which the heads of 

 the pickets were placed in the plane of the base, which frequently was the cause of 

 the planes of the register-heads being out of the direction of the hypotenuses. 

 In this operation however the bottoms, as well as the tops of them, were placed 

 in the true vertical by means of the transit-instrument, and therefore it was not 

 difficult to bring the planes of their tops into the required position. 



For the purpose of using the transit as a boning telescope, as well as an in- 

 strument for taking the angles of elevation or depression, Mr. Ramsden pro- 

 vided 2 mahogany boards, one of which was fastened to the register-head, and 

 the other, furnished with levelling-screws, rested on it, the transit-instrument 

 being placed on the latter. The level belonging to the transit was then hung 

 on the arms; and if the axis proved to be horizontal, which it would be if the 

 brass heads were rightly placed, the instrument required no further adjustment; 

 but if that did not prove to be the case, the axis was made parallel to the 

 horizon by the screws of the levelling-board, which were turned in contrary 

 directions, having in the first instance been worked till within half the limits of 

 their adjustment. By this means the axis was kept at a constant height from 

 the brass heads. 



The method of determining the angles which the measured lines made with 

 the plane of the horizon was as follows. After the hypotenuse was mea- 

 sured, the transit-instrument with its boards were placed on the picket, and the 

 levelling-screws moved if the axis did not happen to be horizontal. The cross 

 board, on which a black line was drawn whose breadth was about twice the ap- 

 parent thickness of the micrometer-wire, and its distance from the bottom of 

 it equal to that of the axis of the instrument from the register-head, was placed 

 on another picket in the hypotenuse, having the brass head which had been 

 before fixed on it still remaining. The telescope was then made horizontal, the 

 index of the micrometer being placed to the zero on its circle, and the wire of 

 the microscope set to bisect that dot on the arch which was nearest to the cen- 

 tre of the field. After this, the telescope was moved in the vertical by the 

 finger-screw, till another dot was bisected, at the same time that the line on the 

 cross board appeared in the glass, by which the angle that the instrument had 

 described on its axis, was measured in half degrees. The remaining part of the 



