Dec. 1, 1881] 
von Baer extended it to the phenomena of rivers ; the above case 
may be considered as connecting together both classes of phens- 
mena. D. WETTERHAN 
Freiburg-im-Breisgau, November 26 
Spectrum of the Electric Light 
WILL you, or one of your spectroscopical contributors, kindly 
inform me in what respects (if at all) the spectium of the electric 
light differs from that of the sun? At a time apparently not far 
distant from the almost universal application of the electric light, 
the question I ask is not unimportant, as it, I believe, affects 
the tolerance of the human eye for other than solar light. It is 
already well known that much work done by gas-light is by 
many found prejudicial to their vision, and this may, | presume, 
be caused by the inherent qualities of the light. It will be in- 
teresting to me therefore to learn in what respects electric light 
and eas-light differ from solar, as shown by spectrum analysis. 
J. Hopkins WALTERS 
Reading, November 28 
A GLIMPSE THROUGH THE CORRIDORS OF 
TIME? 
ile 
AS the remote epoch of which we are speaking the solar 
tides were very small, as they are at present. Yet, 
small as they are, there was a particular circumstance 
which may have enormously increased their importance. 
The point to which I refer can be illustrated very simply. 
We have here a weight of 14 lbs. freely suspended, and 
here I have a small wooden mallet which barely weighs 
half an ounce, yet small as this mallet is, I can make the 
heavy weight swing by merely giving it blows with the 
mallet. Let metry. I give the weight blow after blow. 
I hit it as hard as I can, yet the weight hardly swings. 
I have not yet been successful. The art of succeeding is 
merely to time the blows properly; this I am now doing, 
and you see the weight swings in an arc which is steadily 
augmenting. 
We therefore see that a succession of impulses, in 
themselves small, can yet produce a great effect when 
they are properly timed. In the present case the impulses 
should succeed each other at the same interval as this 
pendulum requires for one to and fro oscillation. The 
time therefore depends on the body struck, and not at all 
on the body which gives the impulses. 
Just as this pendulum swings with a definite period so 
the vibrations of the primzval earth had a certain period 
appropriate to them. Suppose that the liquid primeval 
globe were pressed in on two quadrants and drawn out 
on the two others, and that the pressures were then re- 
leased. The globe would attempt to regain its original 
form, but this it could not do at once, any more than the 
pendulum can at once regain its vertical position; the 
protruded portions would go in, but they would over- 
shoot the mark, and the globe would thus oscillate 
to and fro. Now it has been shown that the period 
of such oscillations in our primitive globe is about an 
hour and a half, or very close to half the supposed length 
of the day at that time. The solar tides, however, also 
have a period half the length of the day. Here then we 
have a case precisely analogous to the 14 lb. weight I 
have just experimented on. We have a succession of 
small impulses given which are timed to harmonise with 
the natural vibrations. Just as the small-timed impulses 
raised a large vibration in the weight, so the small solar 
tides on the earth threw the earth into a large vibration. 
At first these vibrations were small, but at each succeeding 
impulse the amplitude was augmented until at length the 
cohesion of the molten matter could no longer resist: a 
separation took place: one portion consolidated to form 
T Lecture delivered at the Midland Institute, Birmingham, on October 
24, 1881, by Prof. Robert S. Ball, LL.D., F.R.S., Andrews Professor of 
Astronomy in the University of Dublin, and Royal Astronomer of Ireland. 
Contributed by the Author. Continued from p. 82. 
NATURE 103 
our present earth ; the other portion consolidated to form 
the moon. 
There is no doubt whatever that the moon was once 
quite close to the earth; but we have to speculate as to 
what brought the moon into that position. I have given 
you what I believe to be the most reasonable explanation, 
and I commend it to your attention. There are difficulties 
about it, no doubt: let me glance at one of them. 
I can easily imagine an objector to say, “If the moon 
were merely a fragment torn off, how can we conceive 
that it should have that beautiful globular form which we 
now see? Ought not the moon to have rugged corners and 
an irregular shape? and ought not the earth to show a 
frightful scar at the spot where so large a portion of its 
mass was rent off.” 
You must remember that in those early times the earth 
was not the rigid solid mass on which wenowstand. The 
earth was then so hot as to be partially soft, if not actually 
molten. If then a fragment were detached from the earth, 
that fragment would be a soft yielding mass. Not for 
long would that fragment retain an irregular form; the 
mutual attraction of the particles would draw the mass 
together. By the same gentle ministrations the wound 
on the earth would soon be healed. In the lapse of time 
the earth would become as whole as ever, and at last it 
would not retain even a scar to testify to the mighty 
catastrophe. 
I am quite sure that in so large and so cultivated an 
audience as that which I am now addressing, there are 
many persons who take a deep interest in the great 
science of geology. I believe however that the geologist 
who had studied all the text-books in existence might 
still be unacquainted with the very modern researches 
which I am attempting to set forth. Yet it seems to me 
that the geologists must quickly take heed of these re- 
searches. They have the most startling and important 
bearing on the prevailing creeds in geology. One of the 
principal creeds they absolutely demolish. 
I suppose the most-read book that has ever been written 
on geology is Sir Charles Lyell’s ‘‘ Principles.”” The 
feature which characterises Lyell’s work is expressed in 
the title of the book, “ Modern Changes of the Earth and 
its Inhabitants considered as Illustrative of Geology.” 
Lyell shows how the changes now going on in the earth 
have in course of time produced great effects. He points 
out triumphantly that there is no need of supposing 
mighty deluges and frightful earthquakes to account for 
the main facts of geology. 
Lyell attempts to show that the present action of winds 
and storms, of rains and rivers, of ice and snow, of waves 
and tides, will account for the formation of strata, and 
that the gentle oscillations of the earth’s crust will explain 
the varying distribution of land and water. In this we 
can to a great extent follow him. I am quite satisfied 
with the oscillations in the land. If the land rises an inch 
or two every century in one place and falls to the same 
extent elsewhere, all that is required has been explained. 
Nor do I feel at present disposed to question his views as 
to rivers or to glaciers, to rains orto winds. There is how- 
ever one great natural agent of which Lyell does not take 
adequate account. He does not attach enough import- 
ance to the tides. No doubt he admits that the tides do 
some geological work. He even thinks they can do a 
great deal of work. The sea batters the cliffs on the 
coasts, and wears them into sand and pebbles. The 
glaciers grind down the mountains, the rains and frosts 
wear the land into mud, and rivers carry that mud into 
the sea. In the calm depths of ocean this mud subsides 
to the bottom; it becomes consolidated into rocks; in the 
course of time these rocks again become raised, to form 
the dry land with which we are acquainted. 
The tides, says Lyell, help in this work. Tidal currents 
aid in carrying the mud out to sea; they aid to a con- 
siderable extent in the actual work of degradation, and 
