366 CARNEGIE INSTITUTION OF WASHINGTON. 



laboratory of the California Institute of Technology. This instrument has 

 been completed, with the exception of the special galvanometer, the design 

 and construction of which were undertaken by the Bureau of Standards under 

 the direction of Dr. Frank Wenner. This part of the equipment has given 

 some difficulty and is not yet available, so that no report can be made at this 

 time upon the effectiveness of the new features which have been incorporated 

 in it. It is designed to record the vertical component of the earth's motion. 

 Independently of this, Messrs. Anderson and Wood made a number of 

 tentative experiments, with the purpose of developing an entirely new type 

 of instrument, using the torsion principle, which is very much simpler than 

 any heretofore used in seismology. It is a great pleasure to report that this 

 work has been crowned with most remarkable success. An instrument 

 of the utmost simplicity, adaptable to the measurement of any one of the 

 three components of movement and capable of detecting short waves as well 

 as long ones, has been perfected in a remarkably short time. Of the first two 

 instruments of this type, which have been in use since January 1923, Mr. Wood, 

 in his annual report, has given the following brief description: 



"A cyhnder of copper 2 mm. in diameter and 2 cm. long is soldered to a 

 tungsten wire about 0.0008 inch in diameter (or to a ribbon made by rolling 

 such a wire flat) so that the wire (or ribbon) is tangent to the cylinder parallel 

 to its axis. A small mirror is attached to the system parallel to the plane 

 through the wire and the axis of the cylinder. The wire (or ribbon) is stretched 

 taut under suitable tension (about 15 grams) between supports placed so that 

 the copper cylinder is held in the field of a permanent magnet strong enough 

 to provide critical damping of the system under the conditions of use. To 

 eliminate 'violin-string vibration,' the wire is passed through two castor-oil 

 drops or films which do not oppose rotation or twisting of the wire appreciably. 



"For registering horizontal motion the system is placed in a vertical 

 position and adjusted until there remains little or no turning moment due to 

 gravity, thus forming a strictly horizontal pendulum with a stable zero position 

 and a restoring moment due wholly or predominantly to torsion. The period 

 is, then, a function, other things being equal, of the lengths of the segments 

 of the wire (or ribbon) suspension. Registration is conducted optically with 

 photographic paper in the well-known way. 



"These instruments may be transported with only ordinary care without 

 injury or serious impairment of adjustment and, if fastened in position, they 

 usually should survive a major earth shock from a near-by origin unharmed 

 and ready to continue or resume registration. 



"This principle is applicable for the registration of vertical seismic motion, 

 as well as horizontal, and the instrument design is practically identical, but 

 the theory of operation is less simple and not so easy to state briefly. 



"The two seismographic recording mechanisms under construction in 

 August 1922 were finished and shop tested, and in December 1922 were 

 installed for trial and experimental operation — one in a basement at the 

 Mount Wilson Observatory offices, under my own charge, and the other in a 

 basement room of the Norman Bridge Laboratory of Physics at the Califor- 

 nia Institute of Technology, in the custody of Professor Walter T. Whitney. 

 Upon trial both soon proved amply reliable and precise for the work required. 



"A slow-motion, eight-day recording system has been developed also, and 

 two have been constructed. These reel a strip of photographic paper 5 cm. 

 wide at the rate of 3 cm. per hour for seven or eight days with one winding of 

 the clocks, thus permitting close, as distinguished from open, registration for a 



