58 REPORTS ON THE STATE OF SCIENCE. — 1915. 



zontally, whicli again refocusses the light into a small intense point. 

 This point falls midway upon a slit -003 in. wide and only the middle 

 portion is permitted to pass to the film, which is in immediate contact 

 with this slit. Perfect definition is produced in this way, with the result 

 that waves of not more than two or three seconds period are shown quite 

 distinctly on paper moving only 8 mm. per minute, thereby securing 

 both high efficiency and economy. 



Special calibrating and adjusting devices were necessary with such 

 high magnification. This has received special attention, and tilts of 1/100 

 of a second of arc can be applied and registered by a beam of light on a 

 distant calibrating scale. AH such operations are performed at a distance 

 from the column, the motion being transmitted by a long flexible cable, 

 so that the inovements of the observer shall not enter into the amplitude 

 shown in the trace. 



VII. The Milne-Burgess Seismograph. 

 {Note by Mr. J. H. Burgess.) 



The Milne-Burgcss machine is a modification of the Milne horizontal 

 pendulum, the chief differences being a magnifica,tion of 100, about 75 

 per cent, of artificial damping, and an increased rate of travel. 



The registration is photographic. A collimator with a 2 in. objective 

 and 21 in. focus is mounted to produce a real image of an illuminated 

 glass rod 1 mm. tliick at a distance of 10 ft. from the object glass. The 

 illuminated line is reflected by means of a piece of sextant mirror attached 

 to the boom just behind the balanced weights through a hole in the wall 

 on to a recording box in another room. In front of the drum a plano- 

 convex cylindrical lens of J in. focus is fixed. Behind this lens is a fine 

 horizontal slit, and in this way the reflected line of light is brought to a 

 focus on the recording drum. The drum has a circumference of 40 in. 

 by 6 in. wide, and travels at the rate of 480 mm. per hour. Accurate 

 time-marks are put on the trace every minute by an electric shutter 

 operated by the Observatory clock. 



The boom has balanced weights and is artificially damped by means 

 of a copper plate attached to the end of the pendulum which moves between 

 four magnets as shown in the sketch. The induced currents produced 

 when the plate moves retards the motion, and in this way about 75 per 

 9ent. of damping is introduced. By the employment of stronger magnets 

 'periodicity could be obtained. 



VIII. Diurnal Wanderings of the Traces. 



The introduction of a high magnification has brought with it an 

 inconvenience in the unsteadiness of the trace. The trace at Bidston, 

 which with the Milne instrument was a series of smooth lines at equal 

 intervals on tlie paper, became at once, with the Milne-Shaw, a series 

 of rippling lines crowded in two places and wide in between, the unequal 

 spacing being obviously due to tidal action, and the ripples, the period 

 of which is of the order of 10 sec, being such as appear to disturb all 

 highly magnified traces. At Bidston these ripples appear definitely 

 to ])e due to wind ; at any rate on a windy day they are largely increased, 

 as will be seen from the portion of the trace for Dec. 4, 1914, given in 

 the ilhistration. The gusts on tliis day went up to eighty miles an hour. 



