50 REPORTS ON THE STATE OF SCIENCE. 



also. This was attributed to the banking up of drainage from the 

 land and the consequent bulging up of the same. It was, however, 

 pointed out by Sir George Darwin that the greater quantity of water 

 in the English Channel might more than counterbalance the effect of 

 the smaller volume in the Solent. 



In the Mersey, as shown by the tide gauges on the Liverpool 

 Landing Stage, the variation in the height of the tide can considerably 

 exceed 10 feet, and in the Dee, at Hilbre Island, the oscillation is 

 practically the same. The difference in the time of high water at 

 these two stations is about half an hour. Consequently, as a glance 

 at the rough map of the coast-line will show, there is a tendency for 

 the load to balance on the east and west sides, while on the north and 

 south, apparently, the difference would be most marked. In these 

 circumstances it is a little difficult to determine what would be the 

 most appropriate azimuth to mount the pendulum, but as the boom in 

 the original seismometer was placed north and south, in the new 

 instrument the direction was made east and west. The seismometer 

 can, however, be turned through any angle if it be felt desirable to 

 continue the investigations. 



The instrument used was designed by Professor Milne and his 

 assistant, Mr. S. Hirota. All the observations were made and discussed 

 by Mr. W. E. Plummer, Director of the Bidston Observatory. 



The boom differs in some essential particulars from the type 

 ordinarily used in the Milne seismometer. It is divided into two 

 parts : one, nearer to the stand, consists of a stout brass rod, carry- 

 ing a weight of about seven pounds. At the extremity of this rod, which 

 is only about 30 inches in length, is placed a light magnifying 

 style, independently earned, and attached to the boom proper by 

 means of a magnetised needle, capable of moving between a slender 

 iron fork. The sensitiveness of the instrument can be increased at 

 will by reducing the distance between the pivot on which the magni- 

 fying style works and the end of the boom. In the original construction 

 the multiplying arm was 10 inches long, rotating about a centre 

 1 inch from the end of the boom, consequently the displacement was 

 magnified ten times. The arrangements for photographing the move- 

 ment were of the ordinary character. The sensitised paper was paid out 

 at the rate of 5 millimetres an hour, so as to make the small amplitude of 

 the oscillation apparent. The tidal displacements were sufficiently notice- 

 able, and the accordance with the time of high water was satisfactory. 

 To increase the sensitiveness of the instrument so as to make the motion 

 more distinct and easily measured, and to remove any danger of the 

 needle failing to engage the steel forks, it was felt desirable to adopt 

 a different method of connection. With this view, Professor Milne sug- 

 gested that the magnetised needle should be removed and the multiplying 

 piece mounted as a bifilar pendulum, an arrangement which allowed the 

 centre of motion to be brought much nearer to the end of the boom and 

 gave a multiplication of about forty times. The method of photographing 

 the point of light was changed, and a thin strip of black paper substituted. 

 This apparatus has been in use since March 1910, and generally works 



