] 34 Influence of Pressure on the Surface Friction of Ice. 



two parallel runners polished with fine emery and carrying 

 a glass tube which could be loaded at will with lead shot. 

 By means of a steel rod, passing through the ^-inch brass 

 tube and suitably bent, the sleigh could be lifted and moved 

 from place to place within the gas chimney. 



A continuous current of freezing-mixture (water with 

 excess of ice and salt) was circulated through the wooden 

 box by a small pump driven off an electric motor. The ice 

 temperature was read by a copper-platinoid thermocouple 

 consisting of one pair of junctions in series with a suspended- 

 coil galvanometer. The second junction was kept in melting 

 ice. Resistance was added to the circuit to reduce the 

 sensitiveness until the deflexion registered 0°'05 C. per scale- 

 division. 



The following procedure was adopted in carrying out an 

 experiment. The sleigh was placed in the gas chimney. 

 The ends were bolted on and the whole was bolted into the 

 wooden box. A large rubber ring formed a water-tight 

 joint between the latter and the w r ooden disk. The sleigh 

 being raised to the top of the tube, distilled water was 

 introduced so as to fill the latter about one-third. This water 

 was frozen solid by the circulation of freezing-mixture. 

 The humped surface of the ice first formed necessitated 

 refloodino- and refreezing before a flat ice surface was 

 obtained. The sleigh was then lowered and the platform 

 tilted until the angle of statical friction was found. The 

 sleigh could be pulled up the inclined ice surface by a fine 

 silk thread passing through the -J-inch brass tube. The 

 sleigh was now raised from the ice and a number of lead 

 shot were introduced. The thermocouple junction was 

 placed amongst the shot. When their temperature had 

 fallen to that of the enclosure, the sleigh was again lowered 

 and the angle of friction was again determined. This 

 process was repeated again and again with the addition of 

 lead shot each time. The angle of friction was found to 

 remain constant until a certain stage of the loading, when it 

 suddenly fell to about half of its original value. It then 

 remained constant for further increases in the load. 



These results, which confirmed those obtained previously 

 with less satisfactory apparatus, are shown in the table below. 

 In the first column is shown the load, i. e., the weight of 

 sleigh + weight of shot added. In the second and third 

 columns are shown, respectively, the coefficient and angle of 

 friction, whilst the fourth gives the temperature of the ice 

 as determined from the galvanometer deflexions. 



