moderately
strong solution of chloride of cobalt in water.

(16) O. C. H. writes: In reply to R. A. R., question 22, in Scientific
American, December 4, I will say that some months ago I was engaged in
running a saw mill, lathe, and shingle factory; was troubled with two
hot boxes, and frequently had to stop and apply ice. Seeing in the
Scientific American a reference to the use of plumbago, I sent for some,
and after three or four applications was troubled no more with hot
boxes.

(17) F. W. asks: What is the best way for return pipe to go into the
boiler from radiators--steam at 60 lb. per square inch, fall 15 feet? A.
If your job is properly piped you can bring your return pipe in at any
convenient place in your boiler below the water line. If you go into the
feed pipe, have your connection inside all other valves.

(18) L. T. G. writes: 1. I have four cells of carbon battery; the
solutions are bichromate of potash and sulphuric acid. Also three cells
of the Smee; sulphuric acid one part, to ten of water; and the four
cells of the carbon battery are not sufficient to run my small
electro-magnetic engine, for more than two or three minutes. I wish to
know if it would be injurious to either one of the batteries if I should
unite them both in one circuit, to run the engine, for about one or two
hours at a time. A. The batteries will not be injured, but they will not
work well together. Better increase the number of carbon elements. 2.
Will either of the above batteries freeze in winter, or will cold
weather affect their working? A. They will not freeze, but it is better
to keep them at a temperature above freezing 3. Is it always best to use
the largest wire in connecting batteries with any instrument, say, above
No. 11 or No. 12 wire, as the larger the wire the less the resistance,
thereby getting nearly the full power of the battery? A. Yes. 4. What
purposes are quantity and intensity electricity best suited for
respectively? A. Batteries are arranged for quantity or intensity
according to the work to be done. The maximum effect is obtained when
the battery elements are combined, so that the total resistance in the
elements is equal to the resistance of the rest of the circuit.

(19) J. H. asks: Which would be the strongest, two 2-inch by 4-inch
joists nailed together, or one 4-inch by 4-inch joist? A. One 4-inch by
4-inch.

(20) J. K. B. writes: I suppose every experimenter who uses a carbon
battery has been t