(7) J. De F. asks: 1. Knowing the resistance of a wire of given conductivity, length, and diameter, will the resistance of any other wire be in proportion inversely? A. Yes. 2. Is there heat enough developed in the secondary coil of an induction coil to prevent the use of paraffine as an insulating material? A. With proper battery power, no. 3. How high in the list of non-conductors does paraffine stand? A. It is one of the best. 4. Will a cotton insulator soaked in paraffine answer as well as silk? A. No, because it renders the covering of the wire too thick. 5. Can you recommend any insulating material for making induction coils which will dry rapidly? A. Alcoholic shellac varnish. Rosin to which a little beeswax has been added is an excellent insulator; it must be applied in a melted state. 6. What is the composition of the black material covering the Leclanche porous cell? A. Gutta percha. 7. Is the magneto-electric machine described in the SCIENTIFIC AMERICAN SUPPLEMENT patented? A. To which do you refer? Most, if not all of them are patented. (8) B. V. F. writes: With reference to item 8, on page 139, of SCIENTIFIC AMERICAN, March 1, 1879, I think there is some mistake about the coal you think required to heat 1,000 cubic feet space. I burn some 8 tons coal to heat, in the whole year, such part of my house as must exceed 25x20x18=9,000 cubic feet. We keep up a moderate heat at night. Ventilate more than most families do; take part only of the cool air, and only in part of the coldest weather, from the cellar, which at such times is opened into the main entries. House wood, back plastered, and stands alone. If 100 lbs. coal would heat 1,000 feet one day, I ought to burn 900 lbs. a day, or nearly 14 tons in December and 14 more in January. A. We are glad to receive these data, which correspond quite closely with some obtained by recent accurate experiments. The estimate given in the SCIENTIFIC AMERICAN also agrees well with experiments on the use of hot air heaters for very small buildings or rooms. Of course, the larger the space to be heated, the more economically it can generally be done. (9) W. M. S. asks: Will the coil described in SUPPLEMENT NO. 160 do for the electric pen described in a recent number of the SCIENTIFIC AMERICAN? If not how must it be changed? A. It is too large; make it one half the size given. [Illustration]