36 BULLETIN OF THE 



posed to carry forward only one tripod for each unit, and to give 

 it uo vertical adjustment. 



To execute this plan a novel rod is proposed and a nunaber of 

 tripods of a novel pattern. 



The Rod. _ The material is not essential. To the under surface 

 at each end is attached a steel sphere one-half inch in diameter. 

 The distance between the centres of the spheres is the unit of 

 length. Some appliance must be attached for the reading of 

 inclination. 



The IVipods. The head of each is broad, and upon it rests a 

 free plate bearing a conical socket to receive one of the spheres 

 attached to the rod. A device is added to lift the plate upon 

 balls at will, and another device to clamp it. 



The Use. Suppose one of the tripods, with clamped plate, to 

 stand so that the conical socket is in the line of the base. Ano- 

 ther is placed in advance at a distance approximately equal ta 

 the rod length, and its plate is undamped. One sphere of the 

 rod is now placed in the fixed socket and the other in the movable. 

 The advance end of the rod is aligned. The balls are lifted 

 under the movable plate for an instant, to substitute rolling for 

 sliding fi'iction, and relieve all strains. The plate is clamped. 

 The inclination of the rod is observed. The advance socket has 

 now become a record of the application of the unit of length. 

 A third tripod is placed on the line and the rod is carried forward 

 to repeat the process. 



The apparatus is proposed with special reference to the needs 

 of such work as that of the Powell Survey. It is hoped that it 

 will combine a high degree of precision with noteworthy rapidity 

 of manipulation. 



The subject was discussed by Messrs. Schott, Abbe, Jenkins, 

 Powell, and Doolittle. 



Mr. W. H. Dall communicated the results of his observations- 

 on 



THE MUSCLES OE THE OYSTER, 



and called attention to the existence of a small anterior muscle,, 

 which he considered to be a pedal one. 



The subject was commented on by Messrs. White and Gill. 

 Mr. C. E. DuTTON commenced a communication on 



the SUCCESSION OF VOLCANIC ERUPTIONS. 



This communication being unfinished when the hour for 



