624 PHILOSOPHICAL TRANSACTIONS. [aNNO 1795. 



On the method of fixing the iron cannon at the extremities of the base on 

 Hounslow Heath, 1791, it is here observed, that as the pipes were found in a 

 very decayed state, and it became certain, were they suffered to remain as the 

 termini, that in a few years the points marking the extremities of the base would 

 be lost, it became necessary to re-establish them in a more permanent manner. 

 Among the various means proposed for this purpose, that of heavy iron cannon 

 was adopted, having been previously sanctioned with the approbation of Mr. 

 Ramsden, and other competent judges. Two guns were therefore selected at 

 Woolwich by order of the Master-General, from among those which had been 

 condemned as unfit for the public service, and sent to Hampton by water. The 

 placing of these guns accurately being an operation of a delicate nature, and 

 attended with some difficulty, on account of their great weight, the mode of 

 performing it was very deliberately considered; and every precaution afterwards 

 taken to render the operation unexceptionable. The method was as follows. 



Four oaken circular pickets, of 3 inches diameter, were driven into the ground, 

 at the distance of 10 feet each from the centre of the pipe, 2 of them being in 

 the direction of the base, and the others at right angles to it. Melted lead was 

 then run into a hollow made in the head of each picket, and afterwards filed off 

 perfectly smooth. On the brass cup, belonging to the r. s., being adjusted in 

 the pipe, silver wires were stretched from the heads of the opposite pickets, and 

 moved till their intersection coincided with the centre of the cup; and in this 

 position a fine line was drawn on the lead of each picket, exactly under and in 

 the direction of the wire. This operation being performed, and the truth of 

 it re-examined, the pipes were taken out of the ground, in doing which it be- 

 came necessary to make an excavation of about 4 feet, in order to clear the cir- 

 cumference of the wheel. It had been at first intended to have inserted the gun 

 so far in the ground as that its muzzle should be even with the surface of the 

 original pipe: but on considering that this was a matter not absolutely essential 

 to the ascertaining of the actual length of the base by any future measurement, 

 provided the axes of the guns were made to coincide with those of the pipes, it 

 was determined to fix the cannon, without digging the pit to a greater depth than 

 that of 10 feet. In this position however it was evident, that the muzzle of 

 the gun would rise higher than the surface of the pickets, which had been put 

 into the ground for finding the centre; which rendered it necessary to drive in 

 and adjust 4 outer pickets, of a proper height, to determine the centre of the 

 bore of the gun, by the intersection of another set of wires. The tops of the 

 first set of pickets were therefore cleared, and the silver wires extended along the 

 fine lines which had been made on the lead. A plummet was then suspended 

 from above, and moved till it fell on the intersection of the wires. Being fixed 

 in this position, another set of wires was stretched across the tops of the 4 outer 



