A. A. A., of Pa.—" Humphrey's Journal," is published in this citj, it is the only publication we know of devoted to Photography. E. P. B,, of Ct —Your improvement in Paddle Wheels is not new. T Y. 0., of N. Y\—If you consult Vol. 6 Sci. Am. on the article Precipitation of Metal, page 83, &c. it will instruct you about engravings formlDg matrices for electropying. L 0., of N. Y.—We do not know the market price of manganese ; it is employed but on a limited scale for the manufacture of bleaching powder, and for coloring glass. W F., of MassYour centrifugal paddle wheel is simply a modification of a blower placed under water ; it is notnew,andhas been employed unsuccessfully for the sume purpose before ; see page 224, Vol. 5, Sci. Am. , W. J. McA , 0. E., N. Y.—We are obliged to you for a copy of your report. You will perceive that we noticed it in a short article, a few weeks ago. F. S. 0,, of Mass.—We have never seen a pump exactly like yours, but there is one embracing the same' principle on page 271 Ewbank's Hydraulics. J. McK , of Pa.—Your name has been entered upon our books for one year, and the back numbers cent as far as we had them. L. W.,of Vt.—A patent was granted to Francis Wolle, of Bethlehem. Pa , for a machine for making paper bagi, and he is a proper person for you to address for the information you solioit of tts. F. S. C, of Boston—We cannot give you an answer with respect to the soda, but the magnesia is certainly injurious to other parts of the system, and consequently to the bones. A. V. G., of Wis.—If the fall of the ram is 10 ft. and the height at which the water is delivered ia 20 feet, the discharge will be one half. E H , of Mass —There are two kinds of machines for rice hulling; the rice is pounded in one and this no doubt breaks, may of the kernels. P. H , of N. J.—Your table has been received, and your remarks are just; but we have noticed that tables are more troublesome to mechanics than simple rules, thus the diameter being 1, the circumference is to it as 314159, and the area is the half of the circumference multiplied by half the diameter. -, of Pa.—It was an oversight, " billion " is a million of millions, and therefore the expression in the instance alluded to was incorrect for billion read one thousand million,—by the French calculation, the expression would have been correct, but not according to our calculation, which follows the English. E. A. J , of Ind—Without entering into an elaborate argument, we must inform you that your calculations are grossly orroneous ; if you n$d sy to; ther proof on thiapoiat wa refer you to articles upon this subject in Vol. 6, Sci. Am. We need not, of course, repeat those arguments when you have such excellent opportunity of examination. A. B. of Ct.—You should always sign your name and give jour address to communications addressed to editors, then they are likely to receive some ttention, otherwise they are not. M. B., of Phila.—Speed does not mean the whole distance travelled but relative only, such as one boat faster than another; this was the reason we mentioned the miles per hour, and had you critically examined the case, you would have seen that we spoke of velocity per hour, not the final time, because it is stated that the increase of fuel was as 6 to 083 tons per hour, which is the faot. L. T., of Iowa—The use of rollers for reducing friction between bearings is an old device and could not be claimed as new. C. E., of Mass.—We are not in the habit of receiving pay for such notices, they are voluntary on our part, and we cannot depart from this, the only pro per mode of conducting a journal, otherwise we are likely to deceive our readers by advocating unworthy subjects. If your alleged invention is new and useful we should not object to giving it a notice. Money received on account of Patent Office business for the week ending Saturday, April 16 :— B. D. G., Of Miss , $20; S B. B., of N. Y, $20; E. L. B., of N. Y.,$55; I. H. B., of Ot.. $25 ; N. N. T , of N. Y., $25; J. G., of N. Y., $80 N. B., of N.Y., $236,68, G. E., of N. Y., $50; 0. C, of L. I., $600, G. B., of Ohio, $35; J. G., of Pa., $55; F. 0. D., of Pa., $60 ; J. W. H., of R. I., $26 ; H. R., of N. J. $27,1 Specifications and drawings belonging to parties with the following initials have been forwarded to the Patent Office during the week ending Saturday April 16 :— S. B. B., of N. Y.; I. H. I., of Ct.; N N. T., of N. Y;D. E. McD.,of N. Y.; D. A. M., of Pa.; G. B. Jr., of N. Y; H.R.of N J. BACK NUMBERS AND VOLUMES—In reply to many interrogatories as to what back numbers and TO-lumes of the Scientific American can be furnished, we make the following statement:—Of Volumes 1, 2 3 and 4—none. Of Vol. S, all but six numbers, price, in sheets, $1; bound, $175. Of vSHHtte 6, all; price in sheets, $2; bound, $2,76 Of Vol. 7, all; price in sheets, $2; bound, $3,75. Of Vol. 8, all the back numbers subsequent to No. 27, but none previous. PATENT LAWS, AND GUIDE TO INVENTORS.—We publish, and have for sale, the Patent Laws of the United States. The pamphlet contains not only the laws .but all information touching the rules and regulation of the Patent Office. Price 121-2 \ cts. per copy. A Chapter or Suggestions, &c PATENTEES—Remember we are always willing to execute and publish engravings of your inventions, provided they are on interesting subjects, and have never appeared in any other publication. No engravings are inserted In our columns that have appeared in any other journal in this country, and we must be permitted to have the engraving executed to suit our own columns in size and style. Barely the expense of the engraving is charged by us, and the wood-cuts may be claimed by the inventor, and subsequently used to advantage in other journals. PATENT CLAIMS—;Persona desiring the claims of any invention which has been patented within fourteen years, can obtain a copy by addressing a letter to this office—stating the name of the pa tentee, and enclosing one dollar as fee for copying G-ITK INTELLIGIBLE DIREOTIONS—Weoftenreceive letters with money enclosed, requesting the paper sent for the amount of the enclosure, but no name of State given, and often with the name of the post office also omitted. Persons should be careful to write their names plainly when they address publishers, and to name the post office at which they wish to receive their paper, and the State in which the post office is located.