TIDAL OBSERVATIONS. 



131 



The following resulting chronometer corrections (AT) of the eight day chrono- 

 meter No. 2007, on Port Foulke mean time, is extracted from the discussion of the 

 astronomical observations of the expedition (Part I). 



June 1,1861 ...... Ar=— 4M'7"52» 



July 10, 1861 _4 4T 15 



Hence daily rate ST= +1M 



With these data we find the corrections A^to the watch as follows: — 



June 6, Ar= — 0".9 



June 21, aT= +20"'.1 



1, 



9, 

 11, 



— 0.7 

 + 11 

 + 1.6 



." 25, 



." 26, 



- " 28, 



+ 7.1 

 + 3.2 



— 2.2 



IT, 



19, 

 20, 



+ 14.6 

 + 17.9 

 + 18.9 



" 30, 



July 1, 



" 2 



+ 11.0 

 + 13.6 

 + 15.4 





Average daily 



rate, June 6 to Ju 



le 21 . 



July 3, aT= +17'".2 



4, 



+ 18.7 



6, 



+ 22.7 



7, 



+ 25.8 



8, 



+ 27.7 



9, 



+ 29.6 



10, 



+ 31.8 



+ 1-4 





+ 2.1 





" " " June 30 to July 10 



The preceding observations, taken at regular intervals near the time of each high 

 and low water, generally sufiice to fix the epoch of the highest and lowest level 

 within five minutes. The readings appear quite regular, and are evidently but little 

 afi"ected by agitation of the surface against which the surrounding ice acted as a 

 complete preventive. The mean time during which the same, highest or lowest, 

 readings are recorded has been adopted for the epoch of high or low water, though 

 in some cases a closer process has been attempted by considering the readings pre- 

 ceding and following. If the anterior and posterior slopes of the wave were the 

 same, the average times of any two equal readings of height wotdd give a. closer 

 determination ; for instance, for. low water, June 6 P. M., we have — 



Readinc 



11.9 feet at ... . 





3'^ 



50" 



11.95 feet at 3" 35" and 4" 10" 



mean, 



3 



52 



12,0 feet at 3 20 4 25 



" 



3 



52 



12.05 feet at 3 10 4 30 



" 



3 



50 



Adopted epoch . . . 3 51 

 On the other hand, if the shape of the wave is unsymmetrical, and this is the rule 

 in our case, we find by attempting the above process that the successive times show 

 a regular progression ; for instance, the low water, June 7 A. M. — 



Readiuf 



40" 

 42 

 47 

 50 



_ 12.1 feet at 4" 



12.15 feet at 4" 30"- and 4» 55" mean, 4 

 12.2 feet at 4 25 5 10 " 4 



12.25 feet at 4 20 5 20 " 4 



Here we have to adopt 4'' 40" as the epoch of low water. 



A graphical process appears to be the best in all cases. Suppose the observations, 

 taken at regular (or irregular) intervals, plotted by rectangular co-ordinates (times 

 and corresponding heights), and a number of parallel level lines ruled across the 

 crest (or trough) of the wave. Halving the length of each of these lines (within 

 the curve) and uniting their middle points by a curve, that curve will generally 

 intersect the wave nearly at right angles, and indicate the highest (or lowest) 

 point in it. 



