18 



HYDROGRAPHY. 



they should be covered with paper. They are liable to be much in- 

 jured by sand if this is neglected ; and when scratched are soon liable 

 to become corroded, permanent injury being the consequence. The 

 magnets used to change the poles, if this course is adopted, should be 

 sufficiently powerful to effect it with a small number of manipula- 

 tions. It appears to me that the result of experiments with magnetic 

 needles would be more satisfactory if this were omitted. Careful ob- 

 servations made on returning to the same points, would give the state 

 of the needle, and although all the results would be rendered compa- 

 rative, yet, with the changes known, they could be calculated, and 

 must be of less amount and uncertainty than the usual mode of pro- 

 curing the direct determination by changing the poles, in order to ob- 

 viate the errors likely to arise from a defective axis. 



TIDES. 



Observations on the tides in connection with the surveys were 

 made. Our attention was also directed to the flow of the tidal wave 

 throughout the great Pacific, for the purpose of examining into the 

 anomalies that it is said to exhibit. On arrival in port a station was 

 established as soon as practicable, a position being chosen where the 

 free action of the tide could be felt, and the tide- staffs erected, con- 

 sisting of two stout poles, 3 inches square, with ratchets to receive 

 the springs of a float 8 inches square, which caused the tide to 

 register itself. One of these floats indicated the rise, the other the fall, 

 — the feet, inches, and fractional parts being legibly marked on the 

 staffs. It was noted hourly both night and day, and recorded. Floats 

 were anchored in the stream, which gave the direction, and there 

 the velocity of the tide was ascertained. These have been embodied 

 in the charts and the general hydrographical information. On 

 several islands, 60 to 80 miles apart, simultaneous observations were 

 made, in order to mark more particularly the flow of the tidal wave. 

 The tidal observations have been projected in the usual form in 

 curves of ordinates, as well as the results given in tabular form, 

 which will enable them to be readily understood and the anomalies 

 to be perceived. I have to regret that, in some of the most inte- 

 resting localities, particularly in the South Pacific, — where so great 

 an interest has been manifested, for some years, relative to the appa- 



