Velocity of Transmission of Disturbances through Sea-water. 509 



order to eliminate any effect depending on the velocity of the impact 

 of the pendulum on the rubber, this arrangement was replaced by 

 another, in which the pendulum consisted of bobs supported by fine 

 wire, the armatures being attached to the bobs. The bobs themselves 

 were two in number, one being of lead and the other of plaster of 

 Paris coated with tinfoil; the effective length of the pendulums 

 was in each case 92 cm., the weight of the plaster arrangement com- 

 plete was 160 grams, and of the lead arrangement 966 grams. The 

 arc through which both pendulums fell was the same, and their 

 periods were approximately equal. The result of about ten experi- 

 ments with this apparatus was that the time- constant with lead bob 

 was 0*0095 sec, and with plaster bob 0*01008 sec, showing an 

 apparently smaller constant for the heavier blow. The energy of the 

 blow was, by the arrangement of the apparatus, simply proportional 

 to the masses ; thus the ratio was about as 966 to 160, or about six 

 to one, and the difference in the time- constants was 0'00058 sec 

 One wave of the tuning-fork, however, was rather more than a 

 centimetre in length, and corresponded sufficiently nearly for the 

 purpose to 0*01 sec, and the difference of the time-constants is there- 

 fore about sixty-thousandths of this, say, less than six-tenths of a 

 millimetre on the smoked glass. This is much too large a quantity 

 to be accounted for by experimental error in measuring the plates, 

 but is not too larg*e to be accounted for in other ways. It has already 

 been mentioned that if metallic surfaces are greasy, the definition of 

 the epoch of contact is chiefly a matter of pressure between the sur- 

 faces. Now the contact with the lead bob of the pendulum was, as 

 far as the lead goes, on a clean surface, because the lead was scraped, 

 but wi&h the plaster bob the contact was between two tinfoil surfaces. 

 Now it was found in the experiments above referred to that a much 

 greater press are has to be exerted to produce " contact " when the 

 surfaces are greasy than when they are clean, and tinfoil is always 

 greasy, and consequently we should expect that, as far as this effect 

 goes, the time-constant would appear longer with the lead bob than 

 with the plaster one. This is contrary to what was observed. On 

 the other hand, the tinfoil on the plaster was slightly creased, and 

 hence, if we suppose that a sensible wave of compression does not 

 begin to travel up the tube till the deformation of the india-rubber 

 has progressed to a certain extent, the effect of the tinfoil creases 

 will be to exaggerate the difference between the time-constants in the 

 same sense as that observed. We conclude, therefore, that, setting 

 aside the difference in the rate of propagation of the waves in the 

 gauge tube depending on their amplitude, we must look to the 

 moving part of the contact apparatus for an explanation. This was 

 considerably heavier than was ultimately used in the actual experi- 

 ments, and the adjustment was nothing like so delicate as we after- 



