and, consequently, the quick apparent revolution of one star about the other. The distance of these stars M. Struve finds, by accurate observations in 1821, to be 4"26. = ART. XXVI.-Account of a curious Electro-Magnetic Experiment. By Professor BARLOW. Exhibited at the London Institution, by Dr. BIRKBECK, in his Lectures on Electro-Magnetism. THE following curious electro-magnetic experiment was exhibited by Dr. Birkbeck, on the suggestion of Professor Barlow, at the London Institution, on the 26th May. A hollow globe of wood, 15 inches in diameter, was first accurately turned, and from the equator towards each extremity of its axis grooves were cut parallel to the equator, at the distance of 44° from each other, like parallels of latitude, and another, rather deeper, groove from one pole to the other, along a meridian half-round. Beginning now at the equator with the middle of a wire, about 90 feet in length, and one-tenth of an inch in diameter, which just fitted the grooves, it was carried round in the successive circles towards each pole, making an abrupt turn from one circle to another along the meridian groove above mentioned. From the point where the wire arrived at the poles, it was carefully bound with silk, and returned back again to the equator, along the same meridian. The two ends of the wire being thus brought together, they proceeded to a little distance from the globe, where they terminated. By this means the effect of the short abrupt turnings of the wire along the meridian towards the poles, is counteracted by wire returning back again from the poles to the equator, leaving thereby only the parallel wires active when the two extremities are connected with the battery. The globe being thus far formed, it is covered with zones, in the usual way, so as to exhibit to appearance a common 15-inch terrestrial globe, the wire being completely hidden. But this covering is so laid on, that, instead of the terrestrial pole coinciding with the poles formed by the wire, the latter is brought into Lat. 75° N. and Long. 76° 40′ W. which is the situation which Mr. Barlow conceives will best agree with the observed bearings of the needle in most parts of the world. Things being thus adjusted, the globe is placed on a large cup, not shown in the figure, near the battery, so as to admit of its being placed in any position, or so as to bring any part to the zenith, without the encumbrance of the usual brazen meridian and horizon. A needle is now suspended over the globe, as shown in Plate VIII. Fig. 2, where a b c is a light piece of brass bent as in the figure, between which is hung the small magnetic needle n s, which turns on its axis a b, in two fine holes at a and b, in the wire; leaving thus the needle free to take any dip; while, by means of the silk suspension cd, it is also free to take any direction; lastly, the needle is insulated from the action of terrestrial magnetism by opposing to it the north end of a small bar magnet NS in the line of the dip. By this means the needle retains its magnetic power, but is under no magnetic influence. The extremities of the curves being now connected with the poles of the battery, the globe immediately becomes strongly active upon the needle, causing it to assume the same dip, and the same direction with respect to the artificial globe, as the actual needle docs in the corresponding part of the earth itself, at least to a very considerable extent. Thus, if we bring the Island of Ascension to the zenith, the needle is found perfectly horizontal, with a slight westerly variation. If we bring London to the zenith, we find the dip about 70°, and 24° or 25° of westerly variation; if the globe is again shifted in position, so as to bring Cape Horn in the zenith, the dip is about 60° the contrary way, that is, with the south end below; and the variation about 30° easterly, and so on with various other places. The purpose of this experiment is to show, that what we have hitherto considered as the magnetism of the earth, may be only modified electricity, and to illustrate, experimentally, the theory advanced by M. Ampere, who attributes all magnetic phenomena to electric currents. LONDON, May 28, 1824. ୮ ART. XXVII-HISTORY OF MECHANICAL INVENTIONS AND PROCESSES IN THE USEFUL ARTS. THE HERE never was a period of the history of Europe more entitled than the present to be called the age of invention. The cultivation of the arts of peace has now become general throughout Europe and America ; and it is a singular and gratifying sight to observe the rapid importation of inventions into England, as the only country where they are likely to receive their commercial reward. This excellent effect arises from that wise provision in our patent law, which grants an exclusive privilege for fourteen years to the importer and introducer of a new invention; a provision which, while it gives this country the advantage of foreign genius, has a tendency also to repay the benefit by the high encouragement which it holds out to other nations. In the following brief account of mechanical inventions, we cannot pretend to lay before our readers accurate details of complicated machinery or of tedious processes; but we shall gain our object if we can convey general views, and thus enable the mechanist to comprehend the spirit of the invention, while we point out to him by proper references the works in which more minute information may be obtained. 1. Mr. Bubbage's Calculating Machinery. The extraordinary machinery invented by Mr. Babbage, and now constructing under the patronage of Government, has excited so much interest in every part of Europe, that we have been anxious to gratify the curiosity of our readers by any details respecting the nature and progress of the machine. Although we have had the advantage of receiving from the inventor himself a verbal explanation of the general principles involved in the mechanism, and of some of the particular constructions which are characteristic of the invention; yet it would be impossible, without the aid of numerous figures, and even of models, to render the machinery intelligible even to a mechanical reader. It may be sufficient, however, to state, that the machine is extremely simple in its construction, and that all its functions are performed with a very slight mechanical power. The machine now constructing by Mr. Babbage is intended to compute tables with four orders of differences, and there will be attached to it an apparatus for punching upon copper, or impressing upon some other soft substances the figures which it computes. The permanency of these impressions may be secured either by stereotyping, or by some other processes. "This machine* computes, in all cases, to the nearest figure, whatever it may be; that is, after the required number of figures are computed, if * These observations are taken from Mr. Baily's account of Mr. Babbage's machine, addressed to Professor Shumacher. See Astron. Nachr. No. 46. the next following figure should be a 5 or upwards, the last figure is increased by unity, without any attention on the part of the operator. But it is not in these mechanical contrivances alone that the beauty and utility of the machine consist. Mr. Babbage, who stands deservedly high in the mathematical world, considers these but of a secondary kind, and has met with many curious and interesting results, which may ultimately lead to the advancement of the science. The machine which he is constructing will tabulate the equation A1uz = c; consequently there must be a means of representing the given constant c, and also the four arbitrary ones introduced in the integration. There are five axes in the machine, in each of which one of them may be placed. It is evident that the arbitrary constant must be given numerically, although the members may be any whatever. The multiplication is not like that of all other machines, viz. a repeated addition, but is an actual multiplication; and the multiplier as well as the multiplicand may be deci mal. A machine possessing five axes (similar to the one now constructing) would tabulate, according to the peculiar arrangement, any of the following equations: A5 uz= =auz+1 uz ▲5uz = auz+2 If the machine possessed only three axes, the following series, amongst others, might be tabulated : These equations appear to be restricted, and so they certainly are. But since they can be computed and printed by machinery of no very great complication, and since it is not necessary (after setting the machine at the beginning) to do any thing more than turn the handle of the instrument, it becomes a matter of some consequence to reduce the mode of calculating our tables to such forms as those above alluded to. A table of logarithms may be computed by the equation Aug = c; but in this case the intervals must not be greater than a few hundred terms. Now, it may be possible to find some equation similar to those above mentioned, which shall represent a much more extensive portion of such tables,-possibly many thousand terms: and the importance that would result from such an equation renders it worthy the attention of mathematicians in general. A table of sines may, for a small portion of its course, be represented by the equation A2 uz c: but it may be represented, in its whole extent by the equation A2uza u 3+1. Now, this is precisely one of the equations above quoted: and if a proper machine were made (and it need not be a large one) it would tabulate the expression A sin. from one end of the quadrant to the other, at any interval (whether minutes or seconds) by only once setting it. It would not be very complicated to place three such machines by the side of each other, and cause them to transfer their results to a common axis, with which the printing apparatus might be connected. Such a machine would, amongst other tables, compute one from the expression A sin. + B sin. 24+ C sin. 3. the utility of which, in astronomy, is well known. In fact Mr. Babbage is of opinion that it would not be impossible to form a machine which should tabulate almost any individual equation of differences. Amongst the singular and curious powers produced by small additions to the machinery, may be reckoned the possibility of tabulating series expressed by the following equation: A2 uz A3uz 2x3 the figures found in the tens place of u3+1 As Uz = 4x the figures found in the units and tens place of uz +1 and many others similar thereto. Again, let the machine be in the act of tabulating any series, a part may be attached by means of which, whenever any particular figure (a 6 for example) occurs in the units place, any other number (23 for instance) shall be added to that and all the succeeding terms; and when, in consequence of this, another figure 6 occurs in the units place, then 23 more will be added to that and all the succeeding terms. Or, if it be preferred, the number added shall be added to the term ending in 6 only, and not to all succeeding ones.” 2. Explosive Engine. An engine of a very remarkable kind is, we understand, about to be brought into public notice; which, if it answer the high expectation of its inventor, may ultimately supersede the use of the steam engine. The patents for England and Scotland are, we believe, both completed, so that we may expect soon to hear the particular details of its construction. At the lower end of a small cylinder is placed a minute apparatus for producing oil gas. As the gas is generated, it elevates a piston so as to admit as much atmospheric air as when combined with the oil gas would render the mixture explosive. When the piston has reached this height, the gas is exploded, and the mechanical force of the explosion is employed to drive machinery. Experiments have, we understand, been actually made with this power, which was employed to force up water to a considerable height. Our readers will no doubt be reminded by this brief notice, of the ingenious invention of the Rev. Mr. Cecil, by which the power is obtained by taking advantage of the vacuum created by the explosion of a mixture of hydrogen and common air. Mr. Cecil suggested in his * * See Cambridge Transactions, vol. i. part 2. |