60 
THE YOUNa SCIENTIST. 
with debris being drawn into his syphonal 
or feeding tube. The snails have not 
cleaned off the confervse on the sides of 
the glass to any great extent, except in 
streaks here and there, where they have 
rasped it partially off with their lingual 
ribbons. We will now envelop the jar in 
a wet towel to within an inch of the sur- 
face of the water. This will reduce the 
temperature of the water, and at the same 
time reduce the amount of light passing 
into the jar. The mermaid-weed will 
throw off enough oxygen to support the 
animal life contained in the ja'r for two 
days, for v?hich length of time we will 
leave it. Before the expiration of two 
days all the conferva on the glass will 
have died and passed into the condition 
of humus. This result is caused by the 
reduction of the light, and the lowering of 
the temperature of the water, caused by 
the application of the wet cloth. The 
snails will now be seen going over and 
over the gravel at the bottom of the jar, 
particularly the Planorbis snails, in search 
of any remaining dead vegetable matter 
and conferva, of which they are very fond. 
The water has now become as clear as 
crystal. Not a vestige of these billions of 
plant organisms is to be seen — all have 
been eaten by the snails and mussel, who 
have converted them into harmless earthy 
excremental deposits. 
We will now go over the ground once 
more, just to see what this first lesson 
has gained us in knowledge. First, then, 
we have the positive proof that all pond, 
river, and rain water contains immense 
quantities of the spores of low and minute 
vegetable organisms, which are ready 
and waiting to be developed into visible 
growths whenever the conditions of 
proper resting places, light, and heat, are 
given them. 
Secondly, that whenever, from excess 
of light and heat, these vegetable organ- 
isms are developed too fast, causing the 
tank to look unsightly, they may be 
overcome by removing the causes that 
produced them ; that is, by decreasing 
the amount of light (not wholly cutting it 
off), and at the same time lowering the 
temperature. 
Thirdly, the utilizing of molluscs in an 
j aquarium, but not to the extent of expect- 
ing them, unaided, to clean the glass of 
all vegetable growths, as has been so often 
stated in books on aquaria. 
Fourthly, the nature of decaying vege- 
tation and its affording food for snails. 
Fifthly, the harmlessness of excremen- 
tal substances, and even their positive 
beneflt in a fresh-water aquarium. 
Sixthly, and above all, the great error 
of piling ivater up instead of spreading it 
out and exposing it on all sides to the 
action of light and heat. 
The iColian Harp. 
T^HE season has come when open win- 
J- dows are pleasant, and we invite into 
our houses the cooling breeze loaded with 
the perfume of flowers. To the fragrant 
air of the garden it is easy to add the wild 
melodies of nature's music, and we pro- 
pose to tell our readers how to do it. 
The Harp of ^olus is supposed to have 
! been invented, or at least greatly modified 
by the famous Kircher, who named it 
after ^olus, the god of the winds. It con- 
sists of a long box, such as that shown in 
the engraving, made of any light wood, 
good clear pine answering very well. The 
length of the box should be the same as 
the width of the window in which it is to 
be placed, and it may be five or six inches 
wide, about two deep. The top of the box 
is strengthened at the ends by nailing or 
glueing an extra piece on it so that it may 
hold more firmly the pegs used for fasten- 
ing the strings, and there is at each end 
a bridge similar to the bridge of a violin. 
As the top of the box forms a sort of 
sounding board, there is a hole cut in it, 
as shown in the figure. 
There may be from one to twenty 
strings — five being a very good number. 
They are stretched over the bridge and 
fastened at each end by means of pins, 
just as is done in the violin. When these 
strings are in unison, and the instrument 
is exposed in the window to the action of 
a gentle breeze, they will emit the most 
agreeable combination of wild and melting 
sounds, changing from one harmonic of 
the string to another, according to the vary- 
ing impulse of the wind, and its unequal * 
