394 Prof. J. A. Ewing. Seismometric Measurements [June 21, 



In comparing these observations with those made during the 1878 

 eclipse, it mnst be remembered that the conditions of observation on 

 the two occasions were widely different. The observations in the 

 West Indies were made at the sea's level, in a perfectly humid atmo- 

 sphere and with the sun at no greater altitude than 19°. Professor 

 Langley, in 1878, observed from the summit of Pike's Peak in the 

 Rocky Mountains at an altitude of 14,000 feet, in a relatively dry 

 atmosphere and with the sun at an altitude of 39°. 



From observations on the transmission of sunlight through the 

 earth's atmosphere (Abney, ' Phil. Trans.,' A, vol. 178 (1887), p. 251) 

 one of the authors has developed the law of the extinction of light, 

 and, by applying the necessary factors, it is found that the intensity 

 of the light during the 1886 eclipse, as observed at Grenada, is almost 

 exactly half of that of which would have been transmitted from a 

 corona of the same intrinsic brightness when observed at Pike's Peak. 

 Hence to make the observations of Professor Langley comparable 

 with those of the authors, the numbers denoting the photometric 

 intensity of the corona in 1878 must be halved. The result appears, 

 therefore, that whereas in 1878 the brightness of the corona was 

 0"0305 of a standard candle at a distance of 1 foot, in 1886 it was 

 only 0*0124 of a candle at the same distance. Several of the observers 

 of the West Indian Eclipse (including one of the authors) were also 

 present at the eclipse of 1878, and they concur in the opinion that the 

 darkness during the 1886 eclipse was very much greater than in that 

 of 1878. The graduations on instruments, chronometer faces, <fcc, 

 which were easily read in 1878, were barely visible in 1886. In 

 explanation of this difference in luminous intensity it must not be 

 forgotten that the 1878 eclipse was not very far removed from a 

 period of maximum disturbance, whereas in 1886 we were approaching 

 a period of minimum disturbance. 



XV. " Seismometric Measurements of the Vibration of the New 

 Tay Bridge during the Passing of Railway Trains." By 

 J. A. Ewing, B.Sc, F.R.S., Professor of Engineering in 

 University College, Dundee. Received June 20, 1888. 



The absolute methods of seismometry which have been developed 

 during recent years in Japan, and have been applied to the measure- 

 ment of earthquakes there and elsewhere, may serve a useful purpose 

 in determining the extent and manner of the shaking to which 

 engineering structures are subject through storms of wind, moving 

 loads, or other causes of disturbance. Existing forms of seismograph 

 are well suited for measurements of this kind, provided the frequency 



