106 
AMERICAN AGRICULTURIST. 
[March, 
envelope-box, in about 15 minutes as good an egg- 
tester as one could wish. Our correspondent is not 
exactly correct when he speaks of its being easy to 
“ see through an egg.” When an egg is fresh, its 
contents allow a certain amount of light to pass 
through them, and the egg is translucent but not 
transparent. With a fresh egg the appearance is 
uniform, an equal amount of light passing through 
every part, but if decomposition has begun, or the 
formation of the chick has commenced, the ap¬ 
pearance when held to the light will be more or 
less clouded or even opaque. One with a little 
practice can detect the difference instantly. Egg- 
testers are made for examining one egg at a time, 
but this simple contrivance will allow of more 
rapid operation. It is wasteful to set liens upon 
unfertilized eggs, and each setting should be ex¬ 
amined after they have been under the hen for ten 
days. Any eggs that will hatch at all, will show it 
by that time in not allowing the light to pass clear¬ 
ly through them. Those that will not hatch may be 
used as food for young chicks. This tester allows 
the inspection to be rapidly made, if several hens 
are set at about the same time, the eggs from one 
may be taken to make up for the deficiencies made 
by removing the infertile eggs from the others, and 
a fresh setting be given to that one. A tester like 
this will also be useful to dealers who now resort 
to the slow process of “candling.”— Eds.] 
JEOTS & CTlLMISo 
'I'lie Doctor’s Tnllis. 
One of my young friends, away off in Oregon, too, 
writes—I was about to say, complainingly—but that is 
not exactly the word,—I should say, regretfully, that I 
have “discontinued ” my “ Talks.” He—Walter A. T., 
says: “In your 
talks you used to 
tell us about 
those tilings that 
every boy ought 
to know, about 
heat and cold, 
and all such 
things, and then 
tell us how to try 
experiments, so 
that we could see 
the things our¬ 
selves. Your ex- 
periment of 
breaking the ink- 
bottle by freezing 
the water taught 
me a great deal— 
I shall never for¬ 
get it, for I then 
learned what I 
did not know be¬ 
fore, that water in 
freezing took up 
1.—CENTER OF GRAVITY OF SLATE, more room. Can’t 
you tell us more 
about such things? ”—His letter is longer, but I would 
just here say that Master Walter and I have both been 
thinking of the same thing. I had not “ discontinued ” 
my talks, but only suspended them. I had all last year to 
do so much about the “ Microscope Club ” that I omitted 
my other “ Talks,” for fear that there might be alto¬ 
gether “ too much Doctor” in your part of the paper. 
Having said all that I cared to say about the Simple 
Microscope, I have of late been considering 
WHAT WOULD BEST SUIT 
the youngsters. I say youngsters, meaning both girls 
and boys, and it is not easy to hit upon things that will 
2.— SLATE BALANCED. 3.— IRREGULAR BLOCK. 
be of equal interest to bo h. My “Talks” have been about 
some matters of fact, some law of nature, which really 
should interest girls as well as boys, but I find that as a 
general thing they do not. Now Master Walter, in his 
letter above quoted, gives me a good subject. He would 
like “ to know what people mean when they talk about 
THE CENTER OP GRAVITY. 
What is the center of gravity, and where is it ?”—In re¬ 
newing my “Talks” I will take my young friend’s sugges¬ 
tion, and try to tell something about the Center of Gravity. 
As much of our comfort and our ability to move about 
depends upon our observing matters relating to it, the 
4.— CENTER OF GRAYITT 5.— CENTER OF GRAVITY 
HIGH. LOW. 
subject should interest girls as well as boys. To answer 
my correspondent’s question in the fewest possible 
words, I can take the answer usually given in the books: 
“ The center of gravity is that point about which all 
parts of a body exactly balance each other.” In a ball of 
iron or ivory, the center of gravity will be at the center 
of the ball. In a wooden ruler, the center of gravity will 
be midway between the two ends, and half-way the , 
width of the ruler, hut if we make the ruler 
of three-fourths wood, and at one end have 
one-fourth its length of lead, the center of D >—- —^ 
gravity will no longer be at the center of the E 
ruler, but we shall have to try by balancing it 
to find out where it is. In some cases the 
center of gravity is easily found, while in others 
it is difficult. Suppose you wish to find the 
CENTER OP GRAVITY OF YOUR SCHOOL SLATE. 
Drive a little tack into the frame at A, at¬ 
tach a string to the tack, and hang up the 
slate. Then hold a plumb line (a small cord 
with a large nail or any other weight will 
answer) at the tack, and draw a mark with the pen¬ 
cil, to show where the line crosses the slate; then put the 
tack at B, hang the slate again, and mark where the line 
falls; remove the tack to D, and repeat the marking. 
The point C, fig. 1, the place where these different lines 
cross, will be the center 
of gravity of the slate, 
and if the slate-pencil 
be placed at this point, 
the slate will balance, 
as in fig. 2. If instead 
of a body of a regular 
form,like a slate, where 
the center of gravity 
corresponds to the 
point where lines 
drawn from the comers 
cross each other, the 
body is irregular, as in 
the piece of board, fig. 
3, you have only to 
hang it up, first by the 
point a. and mark a 
line. Then by the point 
c, and mark where the 
_ balanced plumb-line crosses the 
BY knives. first line, and j-ou will 
have the center of gravity at e ; if a little hole were made 
at e, and a pin or other pivot put through it, the piece 
of board would keep its place in any position. For a 
body to stand firm and steady, its center of gravity 
MUST BE AS LOW AS POSSIBLE. 
Take an oval board, for example, as in fig. 4, the center 
of gravity being at c. It will be very difficult to make it 
stand, as in the figure at the left hand, its center of grav¬ 
ity tending to fall. On the other hand, when placed as 
in fig. 5, the piece will stand very firm. While the least 
movement will upset fig. 4, if fig. 5 be disturbed it comes 
back at once to its place. The center of gravity is not 
always within the body, but may be at some point out¬ 
side. Thus in a ring, the center of gravity is at a point 
within the circle. By a knowledge of this fact, some ex 
periments may be made which, at first sight, are rather 
puzzling. In the old trick of balancing a cork by means 
of two forks stuck into it, at opposite sides, or a similar 
one with a stick and two knives, as in fig. 6. the center 
of gravity of the whole affair, the stick and knives to¬ 
gether, is somewhere below the tip of the finger. An¬ 
other form of this experiment is given in fig. 7. This is 
a curved piece of wood with a straight portion at the cen¬ 
ter, the ends being made heavy by means of pieces of 
lead: the center of gravity in this case is between the 
heavy ends, and this allows it to be balanced in the 
singular manner shown in the engraving. Having said so 
much about the center of gravity, I will give you some 
other experiments which are easily performed, and let 
you explain them for yourselves. Fig. 8 shows how a 
pail of water may be hung upon a light stick, laid upon 
the edge of a table, A. The stick, D, C. has a notch cut in 
it at E, and another stick, E, G, has one end caught in 
this notch, with the other 
end resting against the bot. 
tom of the pail at G. The 
pail may now be filled with 
water and still remain in 
place. This experiment may 
be varied, as in fig. 9, by 
using a 56-lb. or other heavy 
weight. The weight is hung 
by a cord, and a rod is used, 
as with the pail of water. It 
may be that the perpendicu¬ 
lar dotted lines will help you 
to an explanation. Figure 
10 shows an old toy, which 
no doubt many of you have 
seen, and it may be used to 
illustrate what seems at first 
a contradiction, i. e., that a body that would fall by its own 
weight, may be prevented from falling by adding weight 
in the direction towards which it would fall. The horse 
by itself, if placed upon the table would fall forward, but 
if we add to it a wire with a heavy weight at one end, and 
bend the wire, it will stand, and even prance in a remark¬ 
able manner on the very edge of the table. There is a 
Fig. 8.—WATER-PAIL HUNG UPON A STICK, 
great deal more to say about the Center of Gravity; 
we wish to talk about Gravity itself and other matters 
concerning it. But this will give my young Oregon 
friend and the rest of you quite as many experiments 
as you can make ready and study upon at one time. 
Xlie Doctor's Correspondence. 
Well, boys and girls, here is something that I did not 
think could be possible 1 I have been asked to explain 
by one who, of all others, I should go to for an explana¬ 
tion. I have had questions from girls, and from boys, 
from one end of the earth to the other; those I accepted 
as a matter of course, but 
A QUESTION PROM “AUNT SUE” 
is something I never looked for, still it is aB welcome as 
it is unexpected. “ Aunt Sue ” here sets a most excellent 
example. While she knows more about “ matters and 
Fig. 10.— BALANCING TOY. 
things in general ” than almost anyone I have ever met, 
the very moment she comes across a thing that she 
doesn't knov> about, she asks. I have always found it so. 
Those who know the most, are the most ready to learn. 
