396 



Mr. T. Proctor Hall on New Methods of 



and 6 the contact-angle. When the plate is wet by the 

 liquid, 



6 = and w = 2(a + b)T. 



When the lower edge of the plate is kept at the general 

 level of the surface, as in fig. 4, the thickness of the plate 



Ffc. 4. 



is of no consequence ; but if it be placed h centimetre too 

 high or too low there will be an error introduced equal to 

 the weight of the liquid supported under the plate or dis- 

 placed by it. That is to say, if p be the density of the 

 liquid and h positive when the plate is too high, an error of 

 ah lip gram is introduced. This error becomes very small 

 without material alteration in w when the plate is made very 

 thin. In practice I found that for a plate 10 centim. long 

 h rarely exceeded +'01 centim ., and that the amount of this 

 error of setting for a plate *005 millim. thick was not more 

 than Jo milligram ; or, for alcohol, about *01 per cent, of 

 the surface-tension. 



Thin plates were placed in a holder (fig. 5) made of glass 



Fig. 5. 



pppp::: 1 



rods 1 millim. thick. Before inserting a plate the holder 

 was balanced so that the stem hung vertically. The spring- 

 clips c (also of glass) were then wedged with little bits of 

 wood, the plate inserted under them, two of the clips released, 

 and the plate adjusted by trial over a liquid until the whole 





