roubled by the uncertainty of the carbon connection.
The makers of the Grenet battery seem to have solved the problem. Can
you tell us through your correspondence column what solder they use, and
how they make it stick? A. The carbon is coated with copper by
electro-deposition; this coating is readily soldered to the carbon
support with common soft solder.

(21) M. D. M. asks: 1. Is there a difference in a steam engine between
the boiler pressure and the pressure on the piston when the piston is
moving 460 feet per minute? A. Yes. 2. About what difference? A. From 2
to 8 lb., depending upon size and length of steam pipe. 3. Does the
difference between them vary with a difference in the motion of the
piston in the same engine? A. Not appreciably within usual limits of
speed.

(22) F. writes: We have just closed up our steam stone works for this
season, and we wish to know what is best to coat the inside of our steam
boilers to keep them from rusting. Some say black oil, and others common
tallow: which do you recommend as the best? A. We think the black oil
quite as good and cheaper than tallow. Have the surfaces thoroughly
cleaned before applying the oil.

(23) O. H. asks for a cheap and easy way of amalgamating battery zincs.
A. It depends on the kind of battery. In the Fuller the mercury is
placed in the porous cell with the zinc. In bichromate batteries all
that is necessary is to dip the zinc in the bichromate solution and then
pour on a drop or two of mercury. It soon spreads over the entire
surface of the zinc. Another method is to dip the zincs in dilute
sulphuric acid and then pour on a little mercury, but these methods,
except in the case of the Fuller battery, are wasteful of mercury. It is
better to apply an amalgamating solution with a brush. This solution is
made by dissolving one part (by weight) of mercury in five parts of
nitro-muriatic acid (nitric acid one part, muriatic acid three parts),
heating the solution moderately to quicken the action; and, after
complete solution, add five parts more of nitro-muriatic acid.

(24) G. W. asks: 1. Would a perfectly round ball of the same specific
gravity throughout lie still on a level surface? A. Yes. 2. Can a
mechanic's square be made so true that a four-inch block may be made
exactly square by such an instrument? A. Yes.

(25) W. H. asks: 1. What is the weight of a boiler 24 feet long, 44
inches diameter, 1/4 inch thick? A. With two flues, 16 inches diamet