C. Barus — Horizontal Pendulum. 501 



Art. XLI. — An Application of the Displacement Inter- 

 ferometer to the Horizontal Pendulum /* by C. Barus. 



1. Introductory. — The displacement of ellipses or of inter- 

 ference lines in the spectrum is probably capable of being 

 photographed for continuous registry, though less easily than 

 the motion of a spot of light. At all events, it seemed inter- 

 esting to endeavor to register the excursions of the horizontal 

 pendulum by displacement interferometry, not so much with a 

 view to recording seismological phenomena as to approach by 

 this means certain other problems as, for instance, the tilting of 

 the earth's surface relatively to the plumb line, the measure- 

 ment of the constant of gravitation, etc. The present paper, 

 therefore, undertakes a new departure with this special end in 

 view, with possibly some ulterior bearing on microseismology. 



If the inclination of the axis of the horizontal pendulum is 

 but a few degrees to the vertical and a large framework is in 

 question (there is scarcely any limit to size other than strength 

 of the material), the sensitiveness of the apparatus, when the 

 excursions are read off in terms of light waves, is astonishing ; 

 or at least, it would be so, if the instrument supplied with 

 mirror and screen had not been so thoroughly perfected. The 

 horizontal pendulum, moreover, has this peculiarity, that it is 

 able to support relatively large weights ; *. e., relatively massive 

 bodies may be subjected to each other's attraction. 



2. Apparatus. — The horizontal pendulum has the usual form 

 of a swinging gate and was constructed of 3/8 inch (vertical) 

 and 1/4 inch (oblique) thin steel tubes. The material available 

 here was unfortunately slightly too thick-walled, a defect 

 which will be modified in the future. Moreover, steel, as has 

 been seen in the work with the electrometer, is an undesirable 

 metal in the varying magnetic field of a city, when the microm- 

 etry of angles is in question. 



The frame of the pendulum, as shown in fig. 1, is very 

 simple. ABC is the truss of steel tube, soldered at A and B 

 and terminating in the brass clutch at (?, into which it is also 

 soldered. The tube AB is slotted at top and bottom and each 

 end receives a solid cylinder a and b of glass-hard steel, snugly. 

 These are held in place by collars c and d. The cylinder b con- 

 tains a conical socket to receive the point of the steel pivot t, 

 a portion of the tube A having been removed at this part. 

 Similarly the cylinder a contains a vertical slot (or reentrant 

 dihedral edge) to receive the pivot s. These pivots are ad- 



* Abridged from a forthcoming Report to the Carnegie Institution in 

 Washington. 



