June 19, 1921 proceedings: philosophical society 285 



secure the state of the tide at places where no predictions are made, it is cus- 

 tomary to refer such places to some port for which predictions are made. 

 The referring of a subsidiary port to the proper standard port has led to a 

 study of the different types of tides, the principal ones of which are the semi- 

 daily, the daily and the mixed. In the semi-daily tide we have two high and 

 two low waters every day, with morning and afternoon tides very much 'alike. 

 In the daily tide we have only one high and one low water a day. In the 

 mixed type of tide, caused by a combination of the two preceding types, 

 we have two low and two high waters a day, but with differences in duration 

 and height of morning and afternoon tides. 



In all types of tides we find a variation with the moon's declination. When 

 the declination is small, the forces giving rise to the daily wave are small, 

 and at such times we may have semi-daily tides even at places where the 

 daily is the predominating type. When the declination of the moon is large 

 the daily forces are at a maximum, and we will have even in the semi-daily 

 tide a difference between the morning and afternoon tides. 



Formerly surveyors and engineers made use of arbitrary or local datum 

 planes. To eliminate the confusion resulting from this practice, which fre- 

 quently made it impossible to correlate the results of recent with the previous 

 surveys, the Coast and Geodetic Survey has been establishing datum planes, 

 based on tidal definition, for the coasts of the United States. The great 

 advantage of the tidal datum plane lies, not only in its simplicity of definition, 

 but also in the certainty with which it may be reestablished at any future 

 time. 



For practical purposes a tidal plane determined from a series of observations 

 covering a period of a month may be considered as well determined. How- 

 ever, such planes determined at dift'erent times may differ considerably, 

 and for the accurate determination, it is necessary to have long-continued 

 observations at selected representative points. At these principal stations 

 the Survey maintains self-registering tide gauges, which automatically record 

 the height of the tide. Such observations covering a number of years bring 

 out changes in mean sea level and furnish the data for a quantitative deter- 

 mination of relative changes in elevation of land and water. 



The space allotted permits the covering of only the practical phases of 

 the tidal work of the Survey as they relate to the needs of the engineer and 

 mariner. The theoretical phases, however, as they relate to the development 

 of the theory of tides, have not been neglected. The tidal papers of William 

 Ferrel and R. A. Harris were results of the vSurvey's tidal studies along these 

 lines. 



The first paper was discussed by Messrs. Sosman, Lloyd and Humphreys. 



D. L. Hazard, Corresponding Secretary. 



847th meeting 



The 847th meeting was held in the assembly hall of the Cosmos Club 

 on February 26, 1921. The meeting was called to order by President Faris 

 with 41 persons present. The program was as follows: 



F. Hastings Smyth and Howard S. Roberts: The system cvipric oxide 

 cuprous oxide, oxygen, (illustrated; presented by Mr. Smyth). 



This system has been studied by several investigators, notably by FooTE 

 and Smith, and L. Wohler has argued rather convincingly that a continuous 

 series of solid solutions exists between the two copper oxides, and that at 



