Height, Length, and Velocity of Ocean Waves. 263 



reference. Especially is this true, since it is rumoured, appa- 

 rently not without foundation, that the Metre des Archives 

 is at present, for some unassigned reason, undesirable as a 

 direct standard of reference. 



It is unfortunate that there is not more general uniformity 

 in the material, shape, and mass of standards of length. 

 Difference in these particulars are fruitful sources of error in 

 comparisons, and when one adds to this the trouble arising 

 from bad defining-lines and imperfect focus, the wonder is 

 that the results are as good as they usually are. It is hard 

 to say what material is least liable to changes, but it is quite 

 certain that substances of crystalline structure, and alloys of 

 which the physical properties are largely dependent on a 

 nearly definite composition, should be avoided. Probably 

 pure platinum, silver, and copper, annealed with the utmost 

 care, and kept for some years before final graduation, are less 

 likely to change than any other material which we know. 

 For short standards, possibly bars of native copper, prepared 

 with as few strains as possible, would give the closest possible 

 approximation to a material which has arrived at a permanent 

 state. 



Physical Laboratory, 



Johns Hopkins University, 



Feb. 22, 1888. 



XXXI. Observations on the Height, Length, and Velocity of 

 Ocean Waves. By Hon. Ralph Abercromby, F. R. Met. 

 Soc* 



THE interest in ocean waves has so much declined in recent 

 years, that physicists have perhaps scarcely realized how 

 much more easily measurements can be taken now than 

 formerly. 



In the old days wave-heights could only be ascertained, more 

 or less, by estimation ; while the length and speed could only 

 be determined by a common watch. Now-a-days the aneroid 

 can easily measure small vertical heights to within one or two 

 feet ; while the fly -back chronograph enables time to be 

 measured to the ,1th second, without taking the eye for one 

 moment off the object to be watched. 



The following observations were taken on board the S.S. 

 ' Tongariro,' in various parts of the 8. Pacific between New 

 Zealand and Cape Horn, in the month of June 1885. 



Height was measured by a 4^-inch aneroid with a very 



* Communicated by the Physical Society : read February 25, 1888. 

 T2 



