is 1/2 in. thick. It is well to slightly choke the tube to
better retain the plaster. The paper used must be unsized so that
the solution scan mingle through the pores.

Two liquids are necessary for the cell. Make a strong solution in
a glass or wooden vessel of blue vitriol in water. Dilute some oil
of vitriol (sulphuric acid) with about 12 times its measure of
water and keep in a bottle when not in use. In making up the
solution, add the acid to the water with constant stirring. Do not
add water to the acid.

The cell is charged by placing the zinc in the paper tube and both
placed into the tin can. Connect the two wires and pour the dilute
acid into the porous cell around the zinc, and then immediately
turn the blue vitriol solution into the can outside the paper cup.

A current generates at once and metallic copper begins to deposit
on the inside of the can. It is best to let the action continue
for a half hour or so before putting the cell into use.

[Illustration: Daniell Cell]

Several hours working will be required before the film of copper
becomes sufficiently thick to protect the tin from corrosion when
the cell stands idle. For this reason it will be necessary to pour
out the blue vitriol solution into another receptacle immediately
after through using, as otherwise the tin would be soon eaten full
of holes. The porous cup should always be emptied after using to
prevent the diffusion of the blue vitriol solution into the cup,
and the paper tube must be well rinsed before putting away to dry.

This makes one of the most satisfactory battery cells on account
of the constancy of its current, running for hours at a time
without materially losing strength, and the low cost of
maintenance makes it especially adapted for amateurs' use. Its
current strength is about one volt, but can be made up into any
required voltage in series. A battery of a dozen cells should cost
not to exceed 50 cts. for the material, which will give a strong,
steady current, amply sufficient for all ordinary experimental
work.

A strong solution of common salt may be used in place of the oil
of vitriol in the porous cup, but is not so good.
--Contributed by C. H. Denniston, Pulteney. N. Y.

** A Home-Made Equatorial [253]
By Harry Clark

The ordinary equatorial is designed and built for the latitude of
the observatory where it is to be used. This is necessary since
the hour axis must point to the north pole of the heavens wh