Velocity of Transmission of Disturbances through Sea-water. 513 



The buoys were anchored with sinkers and attached to the pile by- 

 ropes, so as to keep them as rigidly as possible in one position. 

 However, at low tides the ropes became slack, and then if a wind was 

 blowing, the shot drifted for a few yards out of the direct line ; the 

 result will be that the second gauge may be affected by a portion of 

 the wave front (if there is one) which is not identical with the por- 

 tion affecting the first. Owing, however, to the end of the mooring 

 line being permanently fastened to the nearest pile, the shot when 

 displaced will practically move in an arc of a circle, with the pile as 

 centre, and hence we have a means of estimating the maximum effect 

 of any amount of drifting. It can readily be seen by drawing the 

 figure that drifting of the shot will have the same effect as diminish- 

 ing the distance between the piles, and hence the times will be rather 

 shorter, and the velocities rather greater, than would otherwise be 

 the case. An error of this kind will not, therefore, be eliminated by 

 any number of observations, as it is always in one direction. The 

 remedy is to take as much pains as possible to prevent its occurrence, 

 and this was always done ; an account of the magnitude of the error 

 which may arise in this way is contained in the third part of this 

 paper. 



Method Adopted to Determine the Distance between two Spikes 

 in Piles driven out in the Water, Berry's Bat, Port 

 Jackson. 



The base-line was measured with a 100 link ^-inch steel riband with 

 adjustable ends, to which was applied a tension equal to 16 lb. The 

 chain was found after careful testing to be equal to the standard at 

 the Government Observatory, Sydney, at a temperature of 78° P. 

 It was tested on a calm, cloudy morning, the temperature being even, 

 and the base-line was measured the same day, the temperature being at 

 the time 61° F. The standard at the Observatory is x £oths of an 

 inch short, it having been measured off with the compensating rods 

 for measuring trig. -base lines. A correction was therefore applied to 

 make up for this error. The chain when tested was supported through- 

 out its entire length, but was suspended when used for measuring the 

 base-line. By careful experiment it was found that this made a 

 difference in length of y^^-ths of an inch, for which a correction was 

 also applied. The expansion or contraction being equal to 0'005 of 

 1 inch for 1° F. for one chain, the distance in chains, multiplied by 

 half the difference between standard and observed temperature, and 

 the result divided by 8, gives a correction in decimals of a link to be 

 added or subtracted, as the temperature is higher or lower than the 

 standard temperature. 



To the handle at the back end of the chain was attached a small 



2 m 2 



