512 Prof. R. Threlfall and Mr. J. F. Adair. On the 



day was 13° C. This result amounts to making the fork correct at 

 17"81° C. An experiment was made to try whether the slight damp- 

 ing of the fork, produced by the friction of the style on smoked glass, 

 caused any appreciable change in the period. A plate of smoked 

 glass was set up so that the style rubbed the glass, and the beats were 

 again counted. If any effect exists it is less than an alteration of 

 frequency corresponding to one complete vibration in two minutes. 



The effect on the observed velocity of transmission of explosive 

 energy at the extreme temperatures of 13'4° and 28° of the tempera- 

 ture coefficient of the fork is 0'065 and 0*15 per cent, respectively, of 

 the uncorrected value, and is, of course, included in the reduction of 

 the observations. 



Measurement of Distance between the Gauges. 



For the purpose of estimating the distance between the piles a 

 spike was driven into the top of each, to serve as a mark for the 

 cross-wires of the theodolite. "No less than three distinct sets of 

 measurements were made for us by Mr. R. J. A. Roberts, Government 

 Surveyor ; this was due to the fact that on one occasion one of the 

 piles was pulled up by a large steamer, which made fast to it in order 

 to swing, and after the pile had been re-driven rather badly it got 

 washed to one side by the heavy wash of another steamer. Finally 

 the pile was moved further away, a better bottom was found, and we 

 had no further trouble. In the intermediate position the pile was not 

 driven quite straight, consequently the distance between the gauges 

 had to be corrected according to their depth. JSTow the gauges were 

 made to follow the tide, and consequently the correction had to be 

 applied to each experiment. For this purpose, some measurements of 

 the slope were made and a table of corrections for what we called 

 immersion was prepared, by whose aid the distance on each occasion 

 was correctly arrived at. The gauges were fastened to the piles in 

 such a way that no appreciable correction has to be applied to Mr. 

 Roberts's distance between the spikes. We do not mean to say that 

 there may not be some small difference arising from the thickness of 

 the drum (about 3 cm.) and accidental straining of the woodwork, 

 but it cannot amount to more than a few centimetres, and will vary 

 from day to day, as probably in one direction as in another. The 

 piles were also very possibly a little bent by the wind and tide ; the 

 amount of this could not be estimated without keeping a theodolite 

 permanently set up, and this we could not do, partly because we had 

 no theodolite of our own. However, this small error is as likely to be 

 in excess as in defect. 



Much more important for the absolute velocity is the displacement 

 of the firing buoy, by wind and tide, from the direct line of the piles. 



