124 REPORT — 1870. 



Eoberts, with prefixed letter to him from 3Ir, J. E. Hilgard, of the United 

 States' Survey: — 



Mr, J. E. HixGAED to Mr. E. Roberts. 



" United States' Coast-Survey Office, 

 Washington, Aug. 12, 1870. 



" I take occasion to inform you that the reading off of another year's tidal 

 " observations at Port Point, by hourly ordinates, is in progress, in pursuance 

 " of your request under date of May 27, and will be sent in a few weeks. 



" For our own purposes we have as yet only read off from the traces the 

 " high and low waters, and prefer to use for the general discussion the results 

 " of nineteen years. 



" Prof. Thomson at first asked for only one year ; the first year proving 

 *' fragmentary, we read and sent an additional year. The observations now 

 " in preparation wiU bo pretty complete. 



" I shall bo much surprised if you can get reliable constants out of even 

 " two complete years. We will endeavour to pro\'ido for the reading of the 

 " hourly ordinates for the whole scries, and will furnish you copies, if you 

 " find it necessary to have more than the additional year, which we shall 

 " send you soon. 



"J. E. HltGARD, 



Assistant U. S. Coast Survey" 



Mr. E. Roberts to Sir William Thomson. 



" 3 Veridam Bidldings, Gray's Inn, W.C., 

 September 9, 1870. 



" I have received the enclosed [preceding] letter from the U. S. Coast 

 " Survey. I have completed the computation of the hourly heights for 

 " fourteen days for Fort Point, and have compared them with the actual 

 " observations. The agreement is remarkably good, the maximum discre- 

 " pancy not being more than three inches, if the actual mean level for each 

 " day is used, instead of the mean level of the year. The larger differences 

 " shown sometimes, when the annual mean level is reckoned from, are clearly 

 " owing to the abnormal state of the atmosphere at those times. Some of the 

 " elliptic diurnal tides are probably sensible for Fort Point and Kurrachce. 

 " In the computation of the heights for Fort Point the following tides were 

 " omitted, — the whole of the long-period tides, and also the semidiurnal X 

 " [cvection] and y. [variation] and quarter-diurnal M S [Hclmholtz luni-solar 

 " quarter-diurnal]. The maximum effect of the latter three does not exceed 

 " 0-06 ft. I consider the agreement very satisfactory. I expect to have the 

 " heights for Kurrachee for 29 days completed by Monday, and which I 

 " think wiU be satisfactory to Mr. Parkes. I have completed those series of 

 " Ramsgate not included in our first Report ; the M S (Hclmholtz quarter- 

 " diurnal luni-solar) tide is 0-33 ft., which is a Httle larger than what I 

 " should have expected from the corresponding Liverpool component. At 

 " Fort Point and Kurrachee it is scarcely, if at aU, sensible, the analyzed 

 " values for each place not exceeding 0-02 ft. I have made no comparison 

 " for Ramsgate, as the differences will in aU probability resemble those of 

 *' Liverpool already compared, although not differing to quite so great an 

 " extent. 



" Edward Roberts." 



The value of the harmonic analysis is illustrated by the doubt which Mr. 

 Hilgard expresses as to the possibility of obtaining trustworthy constants 



I 



