sease will be weighed and paid
for at the rate of $2.50 per ounce of 25 grammes (about 6-7 of an ounce
avoirdupois). Silk culturists are advised not to attempt the production
of eggs unless they are adepts at the industry, and have had at least one
season's experience. We would advise each person desiring to sell, to
send a sample first, with a statement of the quantity offered.

* * * * *

Dr. Zintgraff of Bonn has taken a phonograph with him to Africa. He
intends to bring home phonograms of the savage dialects which he will
hire the natives to speak into the machine.

* * * * *

[NATURE.]

DETERMINING THE MEAN DENSITY OF THE EARTH.

In _Nature_ for March 5 (p. 408) Prof. Mayer suggests an improvement in
our method of determining the mean density of the earth, from which it
appears that our plan has not been properly understood. This
misunderstanding, no doubt, has arisen from the incomplete description of
our method given in the _Nature_ (Jan. 15. p. 260) report of the
_Proceedings_ of the Berlin Physical Society, which report was probably
the only source of information accessible to Prof. Mayer. We are led
therefore to give a short description of our method.

Let H I K L represent a section of a cubical block of lead, about two
meters in the edge, and weighing 100,000 kilos. The balance, A B C, is
placed in the middle of the upper horizontal surface. It bears the
scale-pans, D and E. Under these scale-pans the block is bored vertically
through, and two other scale-pans, F and G, are suspended below the
block, attached to the balance by means of rods passing through these
openings.

A weight D is brought into equilibrium by weights in G. The weight in D
is acted upon by the earth's attraction + that of the block, and that in
G by the earth's attraction - that of the block. The weights in G are then
greater than that in D by twice the attraction of the block. The weight
in D in now removed to F, and counterbalanced by weights in E. The weight
in E will be less than that in F by twice the attraction of the block.
The difference of the two weighings gives therefore four times the
attraction of the block. A correction must be introduced for the
variation in the earth's attraction due to the different heights of D, E
and F, G.

[Illustration]

In order to obtain as great a deflection of the balance by the method
suggested by Prof. Mayer, e