JAN. 19, 1921 PROCEKDINGS: PHILOSOPHICAL SOCIETY 37, 



PROCEEDINGS OF THE ACADEMY AND AFFILIATED 



SOCIETIES 



PHILOSOPHICAL SOCIETY OF WASHINGTON 



835th MEETING 



The S35th meeting was held at the Cosmos Club, April 24, 1920, with 

 President Sosman in the chair and 40 persons present. The program was 

 as follows: 



W. H. SouDER and C. G. Peters: Physical properties of dental materials 

 (presented by Mr. Souder). This paper, which was illustrated by lantern 

 slides, has since been published in full in Dental Cosmos for March, 1920. It 

 was discussed by Messrs. A. W. Gray, White and Humphreys. 



H. A. Marmer : Results of recent tidal current investigations. 



In the past few years, current obser^-ations have been secured from the 

 Light Vessels stationed on the Atlantic and Pacific coasts of the United 

 States, from five to forty miles offshore. The instruments employed were 

 necessarily of the simplest character. A log line, a fifteen-foot current 

 pole, a sandglass or stop watch, and a pelorus constituted the whole outfit. 

 Observations were made hourly, both night and day. 



The results of these observations show that on the Atlantic coast, in the 

 inland waters and close inshore along the coast, the tidal currents are of the 

 simple rectilinear or reversing type; that is, the flood runs for a period of 

 about six hours in one direction and the ebb for a like period in the opposite 

 direction. Also, the two floods and the two ebbs of the tidal day are very 

 nearly alike. The curve of velocities, therefore, is approximately a sine 

 curve and the times of slack, and of flood and ebb strengths bear a nearly 

 constant relation to the times of local high and low water. 



Farther ofl"shore, the currents on the Atlantic coast become more com- 

 plex. Instead of flowing in the same general direction for a period of six 

 hours, and in the opposite direction for a like period, the current constantly 

 changes directions, clockwise, at such a rate that in a little over twelve 

 hours it will have veered completely around the compass. That is, the 

 current is rotary instead of being rectilinear. In a tidal day, therefore, the 

 curves representing the velocity and direction of the current will appear 

 as two ellipses, bearing a close resemblance to each other in every respect. 



The rotary current is distinguished from the rectilinear current not only 

 by the constant change in direction, but also by a difference of behavior 

 in the change of velocity. In the rectilinear type, the current starts from a 

 slack or period of no motion and gradually increases until it attains a max- 

 imum, after which it decreases to another slack. In the rotarj?" current, 

 there is no period of slack; the current is running at all times. There is, 

 however, a maximum and a minimum velocity, corresponding to half the 

 major and minor axes of the ellipse representing the curve of velocities. 

 This rotary type of current having a circuit of approximately twelve hours 

 and the two circuits of the day being very much alike, is typical of the off- 

 shore current of the Atlantic coast of the United States. 



