1888.] On the Plasticity of Glacier and other Ice, 343 



ends projecting slightly. In the later experiments we used pieces of 

 glass tube or rod for needles, to obviate any possible exaggeration of 

 the extension through the needle bending in the ice. The glass had 

 the further advantage of being a bad conductor of heat. We found 

 that, when air above freezing point entered the chamber during the 

 taking of a reading, the steel needles were apt to work loose, although 

 the body of the ice had not time to materially rise in temperature. 

 Such readings are of course discarded. The two conical collars were 

 filled with ice by freezing water therein. The upper collar was taken 

 out and inverted, and its brass plate levelled. Then the bar was 

 carefully placed in a vertical position and frozen on. The bar was 

 next hung in position in the chamber and frozen on to the ice on the 

 lower collar in situ. 



In the first experiment the measurement of the distance between 

 the upper and lower needles was made with a cathetometer. On the 

 two ends of each needle were glued pieces of paper, on each of which 

 fine ink cross lines had been drawn. The cathetometer was not of 

 the ordinary construction and merits a short description, as, though 

 in practice it was not very successful, in principle it has, we believe, 

 several advantages over the ordinary form. The stand consists 

 of a vertical rod supported by three levelling screws. On this rod 

 slides a metal block, provided with a clamp and slow-motion screw. 

 The telescope rests on this block, being movable through ninety 

 degrees about a vertical axis. The bearing of the telescope is the 

 only mechanical part of the instrument that requires special care. 

 For the cross wires of the ordinary telescope is substituted a micro- 

 meter scale. The millimetre scale is fixed on a separate stand as near 

 as possible to the bar of ice and at the same distance from the telescope 

 as the ice is, and is left untouched during the observations, so that it 

 is of no consequence, for measuring small extensions, if it be not 

 quite parallel to the direction of the tension. The distance from the 

 telescope to the ice or to the scale was about 30 cm. On the top of 

 the telescope is fixed a level. We carefully adjusted this, so that 

 when the bubble was at its zero the axis of rotation of the telescope 

 was in the vertical plane at right angles to the tube of spirit. Then 

 if the bubble remained in its central position in every azimuth of the 

 telescope, we could be sure the axis of rotation was vertical. 



The observation was taken by reading the position, on the micro- 

 meter scale, of the image of the mark on the needle, then swinging 

 the telescope round and reading the position, on the micrometer scale, 

 of the two nearest divisions of the millimetre scale. By interpolation 

 the exact height, on the millimetre scale, of the mark on the needle 

 was then readily found. It will be noticed that the cathetometer need 

 only remain steady while the telescope is swung round from the 

 needle to the scale ; whereas in the ordinary form there is a danger 



