DEVELOPMENTS IN ASTRONOMY—PLASKETT. 257 
sea level, the amount of matter inclosed is the same in each. This 
is called the isostatic layer and acts as if it were floating in equili- 
brium on a liquid at that depth. The comparatively new science 
of seismology—in which our observatory is so ably represented by 
Dr. Klotz, whose method of recording the earth disturbances is, I 
may say, now being extensively copied—on the other hand shows, 
from the form and velocity of propagation of earth disturbances, 
that the interior must be about as rigid as steel. This is further 
corroborated by measurements, by a kind of seismograph, of the 
deformation of the solid earth by the luni-solar attraction, which 
in the sea produces the tides but which also sets up, though, of 
course, very much reduced in magnitude, a similar effect upon the 
land. The more recent advances in seismology have been in the 
direction of improving the sensitiveness of the instruments and the 
methods of discussion of data, so that we may hope to gradually 
obtain definite knowledge of the density, rigidity, and elasticity of 
the interior, layer by layer. 
The increase of data in terrestrial magnetism, on the other hand, 
seems to have complicated rather than simplified the problem, which 
is, of course, naturally the case when the fundamental underlying 
cause or principle is unknown; and this, it must be confessed, is the 
case in this science. There can be no doubt, however, of its ultimate 
solution; and, indeed, we are beginning to see some glimmerings of 
light in the magnificent work being carried on at the solar observa- 
tory on Mount Wilson, where one of the most recent and wonderful 
results has been to definitely prove that there are magnetic fields in 
the neighborhood of sun spots. That changes in the terrestrial mag- 
netic elements and solar activity are in some manner connected has 
long been inferred from the frequent, nearly coincident appearance 
of violent magnetic storms following the central transit of prominent 
sun spots. 
We pass naturally, then, from the earth to the sun, to us the most 
important heavenly body, as on it is dependent all life on our planet. 
Very great advances have been made in recent years in the study of 
the constitution of our luminary, and a great deal of attention is now 
being paid to researches in this most important branch of astronomy. 
The International Union for Cooperation in Solar Research, a society, 
or rather group of societies, which was organized about five years ago, 
and which embraces the workers on the sun in all civilized countries, 
has done much toward unifying and rendering effective the great 
amount of material collected. A meeting of this society, which I 
had the honor and privilege of attending as representing this observ- 
atory and also our Royal Astronomical Society, was held last sum- 
mer at the Mount Wilson Observatory, at which many important 
38734°—sm 1911——17 
