144 BULLETIN OF THE 



of the atoms themselves. We may assume the amplitudes of the 

 aetherial waves at their origin, to be a faithful transcript of those 

 of the atomic excursions which generate them : and we must con- 

 clude the latter to be — even in the velocities of the highest in- 

 candescence, extremely small fractious of the length of the resulting 

 waves. For although the amplitude of the atomic orbit represents 

 but the square root of the brillancy, we may reflect that this latter 

 form of energy presents an enormous range of variation. The 

 light from Sirius — for example, supposing it to be in time twenty 

 years in reaching us, — has but 1 — 1,315,000 part of the amplitude 

 of terrestrial sun-light ; the amplitude being inversely as the dis- 

 tance travelled.* And there are among the visible stars doubtless 

 some a thousand times more distant yet than Sirius. 



According to the estimates of Wollaston, and of the younger 

 Herschel, lights may vary in brilliancy forty thousand million 

 times, representing a difference of amplitude of two hundred thou- 

 sand times. To suggest some approximate idea of the form of such 

 aetherial waves, we may liken them to earthquake waves transmitted 

 across the surface of the ocean at the rate of six miles in a minute, 

 which, while leaving on the tide-gage their registered amplitude of 

 15 inches, have for their length 150 miles : being accurately meas- 

 urable waves presenting the ratio of one inch to ten miles.f 



*As the bright sun Sirius is considerably larger that our sun, and prob- 

 ably intrinsically brighter as well, the figure 1,315,000 (representing its dis- 

 tance in units of sun-distance) would be somewhat reduced as a measure of 

 relative wave-amplitude. If the intrinsic splendor of the two suns be the 

 same, the distant one has about 64 times the surface, or eight times the 

 diameter of our own. The probability of greater density in the former — 

 from greater mass, — is offset by the probability of correspondingly higher 

 temperature. Hence assuming the mean densities to be nearly the same, 

 the gravitative pressure of equal gaseous masses on the photosphere of 

 Sirius, would probably be in the neighborhood of eight times that upon 

 our sun, or some 200 times that upon the surface of our earth. 



f The earthquake which destroyed the city of Simoda, in Japan, in De- 

 cember, 1854, generated such a system of waves, which crossing the Pa- 

 cific Ocean, over a distance of 4,500 miles, in the time of 12 hours and 36 

 minutes, left their record on the tide-gages of the Coast Survey, at San 

 Francisco, as having a maximum amplitude of 18 inches. The height of 

 the ocean wave at its origin was, of course, much greater than this. 

 (Smithsonian Report for 1874: pp. 216, 217.— A Lecture "On Tides," by 

 Prof. J. E. Hilgard. (at present Supt. of Coast Survey,) delivered before 



