ON SEISMOLOGICAL INVESTIGATION. 



143 



This is held up to the light of an oil lamp or an ordinary gas-burner 

 at a distance of 30 inches for 3 to 10 seconds. Next it is developed in a 

 little fresh but dilute developer. If the developer appears too strong, 

 add water and a few drops of a 10 per cent, solution of bromide of 

 potassium. Too long exposure causes the parts which should be white to 

 become grey. A weak acid bath (citric acid 1 part in 40 of water) tends 

 to remove stains. In warm climates a saturated alum bath may be used. 

 If blisters appear weaken the hypo-bath. 



Although photographic reproduction is here referred to, reproduction 

 by tracing is quicker and usually sufficient. 



TJie Photograms. — When the pendulum is at rest the photogram con- 

 sists of two straight lines, one of which is thin and the other thick, like 



Fig. 4. — Commencement and Growth of a Tremor Storm. 



'O-P-M. II. P.M. 







ISlvrs. 



those shown over a distance of about a quarter of an inch on the left-hand 

 side of fig. 2 (' British Association Report,' 1896, fig. 19, p. 229,') which is 

 the diagram of an earthquake recorded at Carisbrooke Castle, in the Isle 

 of Wight, but which had its origin in Japan. The reason that two spots 

 of light are used is that for slow movements the fine line gives the best 

 definition, but for rapid movements the light passing through the fine slit 

 is not sufficient to produce an impression on the photographic surface, 

 and therefore, as in the middle of the figure, we have to rely on the image 

 from the large spot. 



Because the watch makes its eclipses at the half-hour the intervals 

 marked as 20 hours, 21 hours, and 22 hours are read as 20-5 hours, 21 "5 

 hours, and 22'5 hours, and then corrected from the known rate of the 

 watch and the observed time of the eclipses. What is chiefly required 



' This figure, like the others, having been reproduced from a wood block, is not 

 so clear as the original. 



