6l6 PHILOSOPHICAL TRANSACTIONS. [aNNO l/QS. 



measured vvitli the steel chain; and though the result does not differ from the 

 glass rods by so small a quantity as General Roy's experiment assigned, yet it 

 does not amount to more than 3 inches on a base exceeding 5 miles. 



The apparatus, provided for the measurement of the base, consisted of the 

 following articles, viz. 1, A transit instrument. 2. A boning telescope. 3. Two 

 steel chains, 100 feet each, with the apparatus for the drawing-post and weight- 

 post. 4. Fifteen coffers of deal, for receiving the chain when extended in a 

 right line. 5. Thirty-six strong oaken pickets of Si and 4-i- feet long; shod, 

 and hooped with iron. 6. Four brass register heads, carrying graduated sliders 

 moved by finger-screws, for adjusting the ends of the chain. One of these re- 

 gisters has a micrometer- screw attached to it, proper for measuring small quan- 

 tities expanded or contracted by the chain. 7- Thirty-six cast iron heads, to fix 

 on the pickets. 



As many of these articles have been described very circumstantially by General 

 Roy in the 75th and 80th volumes of the Philos. Trans, it will only be necessary 

 here to give a flescriplion of the transit-instrument, boning telescope, and the 2 

 new chains; and first of the transit-instrvmient. 



This instrument made by Mr. Ramsden, may be considered as a transit com- 

 bined with a telescopic level, which makes it serve 2 purposes; one for deter- 

 mining points in the same vertical plane; the other to show how much a 

 measured line deviates from the level. It consists of a telescope about 18 inches 

 long, with an achromatic object-glass of about l^V inches diameter. The teles- 

 cope passes through an axis in the manner of a transit, and as it must be used 

 for viewing objects at very different distances, the images from the object-glass 

 will vary in the same proportion; it therefore becomes necessary to vary the dis- 

 tance of the wires, so that they maybe exactly in the same place with the image. 

 For this purpose there is a pinion, moveable by turning a milled head, by which 

 the small tube, with the wires which are contained in the box, are made to ap- 

 proach, or recede from the object-glass. The 2 pivots, or extremities of the 

 axis, are made with great accuracy to the same diameter; and they turn in angles 

 in the uprights. Each of the angles is fixed in a slider; one to move horizontally, 

 by turning a finger-screw; the other vertically, by turning another finger-screw. 



The level is suspended by its hook on the transverse axis. Its use is to show 

 when that axis is horizontal; and it is furnished with an adjusting-screw, by 

 which the 2 hooks may be made exactly of the same length, so that the axis on 

 which it is suspended may become parallel to a tangent to the middle of the glass 

 tube. This level also serves to set the line of collimation in the telescope hori- 

 zontal; for which purpose there are 2 pins attached to the side of the telescope, 

 parallel to its axis: one cf these pins is furnislied with an adjusting-screw, by 



