ON SEISMOLOGICAL INVESTIGATIONS. 57 



the origin exceeding 90°, PR is often reckoned as P ; and it seems at 

 least possible (see Section X.) that some hitherto unrecognised phenomenon 

 is sometimes mistaken for S. This was foreshadowed in the last Report 

 from the fact that, though the magnitude of S is usually much greater 

 than that of P, so that its identification might be presumed to be much 

 easier on the whole, nevertheless the probable error of the observed time 

 of arrival of S is considerably greater than that of P. If some other 

 phenomenon is liable to be mistaken for S, this larger probable error is 

 easily explained. Briefly, the proper interpretation of the seismograph 

 trace is essentially dependent on a proper identification of the times of 

 arrival of P and S ; so that for the present attention may be profitably 

 concentrated on this problem. It is important to increase the magnifica- 

 tion considerably, because the waves of P are particularly small and 

 difficult to pick up ; further, it becomes necessary to provide such magnifi- 

 cation that these small movements shall not be blotted out when damped. 

 The importance of damping is second only to magnification, for probably 

 much of the uncertainty in reading S is due to the inability of free pen- 

 dulums to record the change in character of the ground movement in this 

 phase. 



As remarked above, two new forms of the Milne pendulum have been 

 constructed for trial by Mr. J. J. Shaw and Mr. J. H. Burgess, and brief 

 notes on the details are given in the next two sections. 



VI. The Milne- Shatv Seistnograph. 

 {Note hy Mr. J. J. Shaw.) 



This is a new type of seismograph with high magnification, combined 

 with absolute damping. The magnification is approximately forty 

 times greater than the standard Milne. 



It is an established fact that only fully damped machines give any 

 approximation to a true record of the ground movement ; moreover, 

 with damping, the various phases, P, Pe, S, Sr, and Max, are much more 

 readily determined ; this will be realised by comparing the undamped 

 records with those of the Milne-Shaw for Nov. 24, 1914, illustrated on 

 Plate II. 



The Milne-Shaw gives a record strictly comparable with that of a 

 Galitzin,* but with this distinction, that the latter depends upon and gives 

 a measure of the velocity, whereas the former gives a direct measure of 

 the amplitude of the ground movement, and does not involve the use 

 of a galvanometer. 



The general principle of the apparatus is to multiply the movements 

 of a short damped boom by reflecting a beam of light by means of a pivoted 

 lens of half a metre focal length. 



The boom (16 inches long) carries a mass of 1 lb., together with a 

 damping vane which moves in a strong magnetic field, and brings the 

 boom to rest after each excursion. 



The outer end of boom is coupled to an iridium pivoted miiTor, which 

 it rotates in an agate setting ; by this means 300 midtiplications of the 

 ground movements are obtained. The definition in the trace is a special 

 feature in this machine. The source of light illuminates a vertical 

 cylindrical lens, and the image created is reflected and refocussed by 

 the pivoted reflecting lens on to a second cylindrical lens placed hori- 



* Compare illustrations Plate II. and page 70. 



