[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 whose elevation above the horizon is equal to the latitude of the observer’s station.  The final adjustment of an ordinary equatorial is very tedious so that when once set up it is not to be moved.  This calls for a suitable house to protect the instrument.  It has been the aim of the writer to build a very simple instrument for amateur work which would be adjustable to any latitude, so easily set up ready for work and so portable that it need not be left out of doors from one evening until the next.

[Illustration:  Instrument for Locating Stars]

The instrument is mounted on a tripod or piece of iron pipe carrying a short vertical rod of 3/8-in. steel.  A rectangular wooden frame with suitable bearings rotates about this shaft.  The frame has also two horizontal bearings carrying a short shaft to the end of which the frame carrying the hour axis is firmly clamped.  By this arrangement of two perpendicular shafts the hour axis may be directed to any point in the heavens without care as to how the tripod or pipe is set up.

The frame for the hour axis is about 12 in. long with a bearing at each end.  The shaft which it carries is 1/4-in. steel, carrying the hour circle at one end, and at the other the frame for the declination axis which is similar to the other, but somewhat lighter.  The declination axis is also of 1/4-in. steel, carrying at one end the declination circle and the pointer at the other.