1885.] 



and the Motion of Glaciers. 



99 



passed through the three square holes, a weight was hung on to the 

 middle block, and the whole left hanging up in the grottQ. Thin 

 plates of cork were interposed between the ice and the wood where 

 the pressure was greatest. At the close of the experiment the weight 

 was removed, and the wooden blocks with the bar of ice removed to 

 the edge of the glacier for examination. The weights used were 

 stones taken from the moraine, the weight being ascertained in each 

 case by a Salter's spring balance. 



As the shear to be observed would probably be small, and some 

 melting, at least of the surfaces exposed to pressure, was to be 

 expected, it was necessary to fix fiducial marks in the substance of 

 the ice. Several forms of wooden and metal plugs and pins were tried, 

 but did not remain sufficiently firmly fixed to make accurate measure- 

 ments possible. I ultimately obtained satisfactory results by boring 

 three small cylindrical holes in a horizontal plane at about the points 

 marked c, d, e in fig. 2 



A small American twist drill about 2 mm. diameter, fitted to a 

 carpenter's brace, made very uniform holes with sharp edges which, 

 under favourable conditions, were very permanent and admitted of 

 accurate measurements being made. Before boring the holes I applied 

 the edge of a steel straight-edge to the ice. This melted a narrow 

 straight groove in the line <z, 6, and by taking care to keep this line 

 in the median plane of the body while boring, it was not difficult to 

 secure that the axes of the holes should be very approximately in one 

 plane, so that no appreciable error would be caused if the surface of 

 the bar melted or bad to be reworked before the straight-edge was 

 reapplied at the close of the experiment, and the displacement of the 

 middle hole noted. My working place on the moraine was not very 

 convenient, and it was not quite so easy as it looked to apply a 

 straight-edge accurately to the edge of the holes on the slippery 

 surface of the melting block of ice, but a little experience taught me 

 the necessary precautions, and in my later experiments the measure- 

 ments agreed very well together and appeared to be quite satisfactory. 



In my first experiments the middle block was rather more than 

 5 cm. thick, the outer blocks about 2*5 cm. The apertures were rather 

 more than 5 cm. square. As it was not necessary to drive home the 

 screws which attached block CD to AB (fig. 1), it was possible to 

 take in a bar 5 cm. deep as well as the cork plates. 



The following preliminary experiments may be recorded : — 



(1.) July 5, bar 5x47 cm., weight 135 kilos. Examined July 9, 

 shear estimated at '05 cm., but measurements not quite trustworthy, 

 melting slight, not measured. Shearing force at beginning of experi- 

 ment 288 grm. per sq. cm. 



(2.) July 9, bar 5x4'5 cm., weight 25 kilos. Examined July 14; 

 the bar had apparently not been quite rectangular ; there had been 



h 2 



