150 



On a Series of Barometrical Disturbances. [Dec. 13, 



Krakatoa excepting that which, is contained in the letter of Mr. 

 Watson (published in "Nature," 6th December, 188S), whose ship 

 was within a few miles of the volcano on the morning of the 

 27th August. He refers to an unusually severe explosion as having 

 occurred at 11 h. 15 m. a.m. local time, which is nearly 45 minutes 

 later than the time, 9 h. 82 in., arrived at in the foregoing discus- 

 sion. The point of the disturbance (as indicated by the baro- 

 grams) which has been taken as the front of the wave is the highest 

 point of the first abrupt rise of the trace, and is perhaps on an 

 average not far from one hour after the first signs of disturbance, 

 the increase of pressure having been very rapid during the interval, 

 but broken into two or three steps or oscillations. During the 

 following half-hour there is usually a large decrease of pressure, 

 succeeded by another abrupt rise lasting about half an hour. Then 

 follow a fall of about an hour, then a rise of an hour and a half, and 

 then a fall of an hour and a quarter. The whole length of the dis- 

 turbance on the time scale is between five and six hours, corresponding 

 to an actual distance of between 3500 miles and 4000 miles. The 

 length of the first main wave of the disturbance is about one hour on 

 the time scale or about 700 miles in length over the earth's surface. 



In the present position of our knowledge of the facts, it can only 

 be surmised that the shock of 11 h. 15 m. a.m. of the 27th August 

 observed by Mr. Watson corresponds to tbe second main feature 

 of the disturbance. That the wave which forms the first feature 

 would have originated at 11 h. 15 m. a.m. is apparently inconsistent 

 with the observed velocities, which it has been shown are remarkably 

 consistent, and indicate without much doubt an origin at 9 h. 32 m. 



A.M. 



The barometric disturbance at Mauritius noted by Dr. Meldrum is 

 said to have begun soon after 11 a.m. local time. The distance from 

 the volcano to Mauritius being about 3450 miles, the wave at the rate 

 of 674 miles per hour would have reached the island in 5 h. 7 m. 

 Taking the great shock at 2 h. 32 m. Greenwich mean time, as before 

 reckoned, the wave would reach Mauritius at 7 h. 39 m. Greenwich 

 mean time, or adding the allowance for difference of longitude, 

 3h. 50 m., the local time would be 11 h. 29 m., which agrees satisfac- 

 torily with the facts as recorded. 



In conclusion, it may be noticed that the sea-waves produced by 

 this volcanic disturbance, assuming the time of its occurrence to have 

 been 2h. 32 m. Greenwich mean time of the 27th August, were propa- 

 gated with an approximate velocity of 480 miles an hour to Mauritius, 

 of 430 miles an hour to Port Elizabeth near the Cape of Good Hope, 

 and 420 miles to Galle, and a somewhat slower rate to Aden. The 

 details of the occurrence of these waves on the coasts of India will 

 shortly be laid before the Society by Major Baird. who has informed 



