TO KNOW THE STARRY HEAVENS 
123 
shadow of Jupiter and becomes invis- 
ible by eclipse. At 12:59 the shadow 
of three completes its transit. At 1:14 
satellite two comes between us and 
Tupiter and begins its transit across 
the disc of Jupiter. It can be seen then 
succession, one, two, three four from 
right to left. The numbers are applied 
to the satellites in order of their real 
distance from Jupiter. Satellite five, 
however, discovered since these, is 
closer to the planet than one. 
Figure 3. The positions and motions of Jupiter’s satellites. Three disappear on 
February 23. The open circle represents Jupiter. 
with difficulty if at all as it is of nearly 
the same color and brightness as Ju- 
piter itself. The shadow of two is still 
on the disc. It completes its transit 
at 2:05. At 2:18 satellite three begins 
to cross the planet, leaving satellite 
four alone visible. This condition con- 
tinues until 3:12, when satellite one re- 
appears. Before completing its eclipse 
the satellite passes behind the planet 
as seen from the earth and is occulted, 
as this is called. Hence it reappears 
after its occultation. At 3 42 the tran- 
sit of two ends and at 4:09 the transit 
of three ends and all four satellites are 
again visible. It is not very frequently 
that three satellites are invisible at the 
same time. 
Figure 3 shows the positions of the 
satellites at 2:30 A. M. February 22, 
February 23 and February 24, with 
lines indicating the motions of the 
satellites in the interval. The posi- 
tions are as they are seen in an invert- 
ing telescope. Satellite one for exam- 
ple first makes a transit on February 
22, then is eclipsed and occulted as 
we have described on February 23, and 
then makes another transit on Febru- 
ary 24. Satellite two moves more slow- 
ly and only makes the transit we have 
described on February 24 in changing 
from the right side of the planet to the 
left. The same is true of satellite three. 
Satellite four moves very slowly and 
has changed its position but slightly in 
the interval. On February 22 the order 
of the satellites left to right is one, two, 
three. On February 24 the order of 
these three satellites is just reversed 
and we have them arranged in regular 
How Much Silver Has Man Dug Out 
of the Earth? 
BY CHARLES NEVERS HOLMES, NEWTON, 
MASSACHUSETTS. 
All of us have heard and read a great 
deal about silver, and all of us are well 
aware that the kind of silver which 
most of us possess does not purchase 
as much of the necessities and luxuries 
of life as it used to. For many years 
in our country silver has not been as 
popular as gold, but it was once a very 
popular metal throughout the world, 
and the “pound sterling,” the British 
monetary unit, was originally an actual 
pound’s weight of silver. It is possible, 
inasmuch as nature is more lavish with 
silver than with gold, that man discov- 
ered and used the former metal before 
he discovered the latter. However that 
may be, gold is mentioned first in the 
Holy Bible in the second chapter of 
Genesis — “The whole land of Havilah, 
where there is gold.” Later, in the thir- 
teenth chapter of Genesis, silver is 
mentioned, indicating that it was val- 
ued highly — “And Abram was very 
rich in cattle, in silver, and in gold.” 
Some of us possess more silver than 
others of us, and annually it is an- 
nounced that each of us should possess 
a certain amount of silver coinage. As 
we all know, silver is much lighter than 
gold, gold being about one and eight- 
tenths times heavier than silver, al- 
though more pockets have holes worn 
in them by silver than by gold. Oc- 
casionally a silver coin will tarnish in 
our pocket — that is, if it stays there 
long enough to tarnish — and this tar- 
