EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 31 



peculiarity, that it is able to support relatively large weights; i.e., relatively 

 massive bodies may be subjected to each other's attraction. 



1 9. Apparatus. The horizontal pendulum has the usual form of a swing- 

 ing gate and was constructed of 3/6 -inch (vertical) and /^-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 micrometry of angles is in question. 



The frame of the pendulum, as shown in fig. 19, is very simple. ABC is the 

 truss of steel tube, soldered at A and B and terminating in the brass clutch at 

 C, 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 6 contains a conical socket 

 to receive the point of the hori- 

 zontal steel pivot t, a portion of 

 the tube A having been removed 

 at this part. Similarly the cylin- 

 der a contains a vertical slot (or 

 reentrant dihedral edge) to re- 

 ceive the horizontal pivot s. 

 These pivots are adjustable to- 

 ward and from the rear, from 

 right to left, and each is revolv- 

 able about a horizontal axis 

 normal to the figure, in a way 

 which will presently be shown. 

 The distance between pivots was 

 97 cm., the distance between the 

 cylinders AB and D about 1 1 1 cm., and the reduced end projects about 16 cm. 

 beyond the edge EE of the brick pier to which the pivots are attached. D, 

 clutched by C, is the hollow stem of the tablet /, which holds the plane dot 

 slot arrangement to secure the grating g, a spring passing down the interior of 

 the tube D. The lower pivot t should preferably point towards the center of 

 gravity G. 



The whole apparatus is inclosed in a more or less triangular flat case h'mnk, 

 firmly bolted to the wall at q, m, and p. The two sides of the case beyond the 

 pier, h'ilk, may be slid off to the left, and then the whole remainder lifted off 

 its bearings without touching the pendulum, as the case has no rear wall. 

 The front face is within 3 inches of the face of the pier. This arrangement 

 was found very satisfactory. The head of the case ki is of course glass-faced 



FIG. 19. 



