514 Prof. R. Threlfall and Mr. J. F. Adair. On the 



piece of steel with a knife-edge, and this was regarded as the end of 

 the chain. This end was worked by means of a slow motion screw 

 attached to a board, on which the chainman knelt, and so the end of 

 the chain was kept perfectly steady. The knife-edge was brought by 

 the screw to the mark, which was a very fine pencil dot on a piece of 

 white paper, gummed on to a flat-headed nail driven into the ground. 

 On the forward end of the chain was hooked a spring balance, which 

 indicated a 16 lb. strain, applied by means of a light straining pole, 

 12 feet long, thus enabling the chainman to apply the strain far more 

 steadily and easily than by any other method at present in use ; and 

 also allowing the full length of the chain to be used on a slope equal 

 to about 1 in 5, the end of the chain sliding up and down the pole. 

 The greatest slope in this case was, however, only about 1 in 10, and 

 only for two chains of the entire length. The forward end of the 

 chain was taken as the position of a small hole, through which a 

 string, with a small plummet attached, passed. This plummet was 

 used merely to indicate the position of the nail, and to keep in adjust- 

 ment a contrivance which gave the position of the transit theodolite 

 (carefully adjusted for the telescopic axis) at right angles to the 

 direction of the chain. The end of the chain was sighted by means 

 of the transit instrument, and the telescope being then depressed, 

 the fine point of a hard pencil was lined on the paper before described. 

 This method does away with any errors due to the use of the plummet 

 (especially in windy weather) for marking the end of the chain on 

 sloping ground. By the use of the instrument the end of the chain 

 can be marked to -^nth °^ arL i ncn - This method has also other 

 advantages for accurate measurement. 



The instrument used was a 5-inch transit theodolite, by Troughton 

 and Simrns, of London, having the circle graduated to 20 seconds. The 

 telescope was larger than, and of much superior quality to, those 

 usually fitted to 5-inch instruments. The plummet was suspended 

 from the end of the vertical axis. The instrument was used on a 

 traversing top stand, and being in very good order, and the axis 

 moving freely, there was no twisting pressure (torsion) on the stand, 

 and the day being a cloudy one, and the sun completely obscured, the 

 elf ect of sunlight on the eyes was avoided. The instrument was care- 

 fully set up over each of the points A, B, C, and D on the base-line, 

 and angles were observed to the spikes in the piles (points E and F) 

 off line AD. The angles shown on the plan are the means of four 

 readings at each of the stations. After the first two readings the 

 telescope was turned over in the reversed position, so that any possible 

 errors due to change of focus (movement of the optical axis of the 

 instrument), want of horizontality of the telescopic axis, and imper- 

 fect collimation, are entirely eliminated. The levels were sensitive, 

 and of good quality. The readings of both verniers were recorded 



