HINTS TO CORRESPONDENTS.

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(1) L. M. W. asks (1) a receipt for marsh mallows, as made by confectioners. A. Dissolve one-half pound of gum arabic in one pint of water, strain and add one half pound of fine sugar, and place over the fire, stirring constantly until the sirup is dissolved, and all of the consistency of honey. Add gradually the whites of four eggs well beaten. Stir the mixture until it becomes somewhat thin and does not adhere to the finger. Flavor to taste and pour into a tin slightly dusted with powdered starch, and when cool divide into small squares. 2. The title of a good veterinary journal. A. American Veterinary Review, New York. 3. The formula for a spavin cure? A. Take of sweet oil 4 ounces, spirits of turpentine 2 ounces, oil of stone 1 ounce. Mix and apply three times per day. 4. A receipt for a wash that will prevent rabbits from injuring the bark of fruit trees. A. We know of nothing good for this purpose.

(2) A. L. K. asks how common fat can be rendered into tallow in an open kettle. A. Keep the tallow melted for some time, along with about two per cent of sulphuric acid largely diluted with water, employing constant agitation, and allowing the whole to cool slowly; then remelt the cake with a large quantity of hot water, and wash well.

(3) W. C. B. asks about the process and kind of machinery used in preparing raw sienna for paint. A. The raw sienna is thrown directly on the hearth of a reverberatory furnace and kept thoroughly raked until it assumes a proper color. Very little, if any, sienna is known to be burnt in this country.

(4) W. M. M. asks for some transparent paint suitable to paint on tracing muslin. A. You must use a transparent varnish such as the following: Dissolve 30 parts of copal and 2 parts of camphor in 120 parts of oil of turpentine and 30 parts of oil of lavender. Use lakes, gamboge, Prussian blue, and the other transparent colors, mixed with the vehicle.

(5) S. S. asks a receipt for black heads. A. Cover the parts afflicted with a pomade consisting of kaolin 4 parts, glycerine 3 parts, acetic acid 2 parts, with the addition of a small quantity of ethereal oil. See Supplement, No. 542.

(6) W. L. asks (1) a cure for frost bitten feet. A. For frost bites, rub the affected parts with pure oil of peppermint. It will also prevent the after effect of chilblains. Care should be taken to use only the pure oil, and not the essence of peppermint, as the essence will not have the desired effect. 2. How plate glass is made. A. See Scientific American Supplement, No. 340.

(7) A. T.—Hard rubber is a very good insulator; gutta percha is also very good, and can be softened by boiling water and given any desired shape.

(8) J. H. S. wants a good receipt to prevent hair coming out. A. Scald black tea, 2 ounces, with 1 gallon of boiling water, strain, and add 3 ounces glycerine, tincture cantharides ½ ounce, bay rum 1 quart. Mix well and perfume. This is a good preparation for frequent use in its effect both on the scalp and hair, but neither will be kept in good condition without care and attention to general health. See articles in Supplements, 102, 388, 396.

(9) A. H. asks the size of steel wire rope necessary to suspend a weight of 16,000 pounds, each end of the rope being fastened 1,600 yards apart, the weight to travel from one end to the other on the rope. A. The scheme of so long a span carrying a load is impracticable. A span of 4,800 feet will nearly absorb the margin of safety by its own weight, depending upon the amount of deflection that could be allowed in the catenary curve. The largest steel cables that are made, 2⅝ inches, weigh 13 pounds per foot, or over 31 net tons for your span; with a deflection of one twenty-fifth, or nearly 200 feet, the tension would be 3¼ times the weight, or 254,800 pounds, while the ultimate strength is but 400,000 pounds.

(10) G. A. L. asks: Why will a brake on the hind end of a train of cars hold more than a brake set ahead of it on train? A. We do not know that it is so. It is possibly a fancy.

(11) A. K. H. asks: Will hot air cool off by sending it rapidly through a wooden tube 300 or 400 feet long? If so, how much? A. Yes, slightly. An iron pipe is better if you wish to cool the air. How much the air will be cooled will depend on initial temperature and the temperature of the conductor and surrounding air.

(12) H. H. writes: I have large quantities of iron and steel to pickle, or, in other words, to clean. I use oil of vitriol, which is expensive and dangerous. Is there an acid that would be cheaper and at the same time as effective? A. We know of nothing cheaper or better than sulphuric acid for pickling castings. The most economical method, as practiced here, is found in the hot bath, a tub lined with lead, or if of small requirement a stone pot. Water 5 to 8 parts, acid 1 part. Boil the work in the acid bath for a few minutes, then rinse in hot water. There is no danger if properly managed. For wrought iron and steel, use hydrochloric acid and water.

(13) W. F. E. asks: 1. How are bath bricks made? A. Bath bricks are found native as minerals, and are imported from England. 2. How are papier mache ornaments moulded, and where can I procure a work on the subject? A. We can send you Spons' "Workshop Receipts" for $2.00, first series, which contains full information on papier mache. See also various articles in Scientific American Supplement on the technology of the paper trade.

(14) W. M. S. asks how to make liquid glue. A. Take a wide mouthed bottle, and dissolve in it 8 ounces best glue in ½ pint water, by setting it in a vessel of water, and heating until dissolved. Then add slowly 2½ ounces strong nitric acid 36° Baume, stirring all the while. Effervescence takes place, with generation of fumes. When all the acid has been added, the liquid is allowed to cool. Keep it well corked, and it will be ready for use at any time.

(15) E. H. F. asks what preparation steam laundries use to make their goods so stiff and have such a fine gloss, and how is it used, and, if used with starch, is hot or cold starch used? A. Melt 2½ pounds of the very best A 1 paraffine wax over a slow fire. When liquefied, remove from the fire and stir in 100 drops oil of citronella. Have a lot of round new pie tins, clean and nice; place them on a level table, coat them slightly with sweet oil, and pour about six tablespoonfuls of the enamel into each tin. The pan may be floated in water to cool the contents sufficiently to permit the mixture to be cut or stamped out with a tin cutter into small cakes about the size of a peppermint lozenge. Two of these cakes added to each pint of starch will cause the smoothing iron to impart the finest possible finish to muslin or linen, besides perfuming the clothes.

(16) J. T. M., Jr., asks for a tempering liquid for tempering a flat coiled spring, ⅛ of an inch thick, 1½ inches wide, 20 feet long, without drawing the temper. A. You can get a spring temper in the hardening bath. Harden in water or oil and draw temper in an iron pan of linseed oil at boiling temperature.

(17) C. E. H. writes: I have nearly 1,000 feet of out-door steam pipe carried in elevated wooden boxes. What is a good and cheap pipe covering to prevent loss of heat? A. Pulverized charcoal or sawdust makes a good cheap insulation for steam pipes. The boxes should be large enough to allow 2 inches clearance all around the pipe, the latter to be retained in position by cleats. Boxes should be tight enough to prevent circulation of air. Tar the outside to make weather proof.

(18) B. L. asks: 1. Is there any method for removing rust stains from white cloth or linen? A. See the table in Scientific American Supplement No. 158, for the "Removal of Stains and Grease Spots." 2. Is there any book showing how to work out chemical problems, and giving answer in the back of the book? A. Professor J. P. Cooke's "Chemical Problems and Reactions" can be sent you postpaid for $1.

(19) D. B. wants a receipt for a dark cherry stain for a white pine floor. A. Use rain water 3 quarts, annatto 4 ounces; boil in a copper kettle till the annatto is dissolved, then put in a piece of potash the size of a walnut; keep it on the fire about half an hour longer, and it is ready to bottle for use.

(20) J. T. S.—Engines with automatic cut-offs will run steady with variable work. If you have a governor that only controls a throttle valve, there will be a small variation of speed with as much variation in the work as you state. Much depends upon the relative amount of work absorbed by the mill and the variable machinery.

TO INVENTORS.

An experience of forty years, and the preparation of more than one hundred thousand applications for patents at home and abroad, enable us to understand the laws and practice on both continents, and to possess unequaled facilities for procuring patents everywhere. A synopsis of the patent laws of the United States and all foreign countries may be had on application, and persons contemplating the securing of patents, either at home or abroad, are invited to write to this office for prices, which are low, in accordance with the times and our extensive facilities for conducting the business. Address MUNN & CO., office Scientific American, 361 Broadway, New York.

INDEX OF INVENTIONS
For which Letters Patent of the United States were Granted
February 8, 1887,
AND EACH BEARING THAT DATE.

[See note at end of list about copies of these patents.]

A Printed copy of the specifications and drawing of any patent in the foregoing list, also of any patent issued since 1866, will be furnished from this office for 25 cents. In ordering please state the number and date of the patent desired, and remit to Munn & Co., 361 Broadway, New York. We also furnish copies of patents granted prior to 1866; but at increased cost, as the specifications, not being printed, must be copied by hand.

Canadian Patents may now be obtained by the inventors for any of the inventions named in the foregoing list, provided they are simple, at a cost of $40 each. If complicated, the cost will be a little more. For full instructions address Munn & Co., 361 Broadway, New York. Other foreign patents may also be obtained.