VOL. LXXXV.] PHILOSOPHICAL TRANSACTIONS. 625 



pickets, till their intersection also coincided with the vertical wire of the plummet, 

 in which position, fine lines were drawn under the wires on the top of each of 

 the outer pickets. The truth of the operation now depending on these last 

 pickets, they were carefully guarded by another set which surrounded each of 

 them, and these last were again bound round with ropes, to preserve the centre 

 pickets from any possible accident. These precautions being taken, and the pit 

 cleared, a large stone of 2^ feet square, and 15 inches deep, containing a circular 

 cavity in its upper surface to receive the cascabel of the gun, was placed in the 

 bottom of it, the centre of the hole being nearly under the intersection of the 

 wires, as determined by a plummet. The gun was then let into the pit, and 

 resting on the stone, it was brought into a position nearly vertical, at which time 

 a quantity of earth and stones were thrown into the pit sufficient to steady the 

 gun. This being done, the cross wires were stretched over the outer pickets; 

 and a pointed plummet suspended from above, having its line coinciding with 

 the intersection of the wires, was let fall into the cylinder, in which a cross of 

 wood that exactly fitted it was placed, whose centre corresponded with that of 

 the bore. The gun was then moved till a dot marking the centre of the cross 

 came directly under the point of the plummet; when earth and stones were 

 rammed round the gun, care being taken to force it by that operation into its 

 proper position, as shown by the plummet and cross. In this manner were the 

 guns fixed at the extremities of the base; and it remains only to be observed, 

 that to prevent the unequal settling of the earth, rammed within the pit, from 

 moving them out of their proper positions, 4 beams of wood were placed in an 

 horizontal direction, having their ends resting against the sides of the pit and 

 the gun. It may also be added, that iron caps were screwed over the muzzles to 

 preserve the cylinders from rain. 



Having, by the re-measurement of the base on Hounslow Heath, sufficiently 

 determined its accuracy, it became necessary, on the approach of the following 

 spring, to form some plan which might enable us to commence the survey with 

 the most advantage. Of those which were suggested, that of proceeding imme- 

 diately to the southward with a series of triangles seemed the most eligible; not 

 only because, in the first instance, the execution of it would forward one great 

 design of the business, in an early determination of some principal points on the 

 sea-coast, but also because a junction of the eastern part of the series with that 

 of the western of General Roy, would afford an early proof of what degree of 

 accuracy had attended both operations. To ascertain the truth of the General's 

 work, by verifying some principal distance or distances, was an object which pre- 

 sented itself, not only as interesting and curious, but as highly necessary, in 

 order to determine whether, by the result, the triangles might stand good, and 

 become a part of the general series. 



VOL. XVII. 4 L 



