Page images
PDF
EPUB

The

which weighs 21 lbs., and presents a very small surface, it is 0.36 for an inch change of the barometer. Now the remedy for this is obvious. If we attach a barometer to the pendulum, its fall transfers a cylinder of mercury from a point near the axis of motion to a greater distance from it; the time of vibration may thus be made to increase by the same amount that it decreases in consequence of the diminished density of the air. By placing the clock in vacuo, as Bessel proposes, (and as Sir James South has actually done for several years past,) the effect of resistance can be determined exactly, and the diameter of tube selected, which will nearly correct it. This is not mere speculation, for Dr. Robinson has verified it by trial. The diameter selected for his tubes (0.1 inch) is not far from the truth. In the autumn of last year a fall of 1.6 inch produced no appreciable change of arc. temperature, however, was then nearly stationary; but notwithstanding its changes during the interval from that time till my leaving Armagh, the arc has been between 1° 36' and 1° 39'. Before the tubes were applied, the limits for the same period were 1° 42′ and 1° 51'. The changes in Bessel's own clock, though made by Kessel, a first-rate artist, are still greater, being from 1° 25′ to 1° 39′, an excess owing in part, probably, to the greater severity of a German winter. From what Dr. Robinson has seen of the vacuum apparatus used by Sabine or South, he cannot refrain from expressing a wish that the experiment were tried of mounting a transit clock permanently in vacuo: such a clock would have many advantages, besides its exemption from changes of barometric pressure.

NEW MERIDIAN INSTRUMENT.

THIS simple and beautiful invention consists of a combination of three reflecting planes, to be used as one single and one double reflector; and in such manner that

an observer may see two images of a distant object when that object is near to an imaginary plane passing through the instrument; and by the coincidence of those images the observer may know when the distant object is in that plane. To render this more clear, and with a view to describe the principles on which the instrument is constructed, let a, c, d, and b, c, represent two rays of light proceeding from two stars and meeting at the point c; and let e, f, represent a transparent surface and reflecting plane perpendicular to the plane a, c, b. If the reflecting plane bisect the angle a, c, b, an observer looking in the direction d, c, will

see one star and the reflected image of the other as if they were both at d. Moreover, if the reflecting plane have an angular motion, or if the plane be supposed to be fixed and the stars to move ás from a and b to a' and b', the reflected image of the star b will appear to move in the opposite direction, or from a to a"; consequently, if the transparent and reflecting plane be fixed in such a position that at some period of the day one star and the reflected image of another may appear in the same place, the following observation may be made:-If an observer look in a proper direction towards the plane, two stars or images will appear to come into the field of view from diametrically opposite points, and move in opposite directions across the field of view, meet at or near the centre, and disappear at the points opposite to those at which they entered; the two images appearing to be in the same place at the moment when that point in the heavens which is midway between the two stars corresponds with the plane of the reflecting surface. The exact position of the observer is not material; for, if he shifts his position, the coincidence of the two images will take place at a different part of the field of view, but at the same time. The recurrence of the coincidence of the two images at intervals of exactly twenty-four sidereal hours will furnish the means of regulating or ascertaining the rate of a clock; and if the time of the coincidence be once known, it will also afford the means of setting the clock or finding its error on any future day. It is to be stated, however, that the reflecting plane must not be placed parallel, or nearly so, to the earth's equator; for if it be so placed, the two images will never separate, or will do so very slowly; the nearer parallel to the earth's axis the plane is, the better, and suitable stars should be chosen accordingly.

The above supposed instrument, described with a view to facilitate the understanding the principles of the invention, cannot be used for observing a single object unless furnished with adjusting screws, and even then it would require nice adjustment, besides being very diffi. cult to fix in a proper position. Fig. 2 will explain the arrangement

[blocks in formation]

of the reflectors in the complete instrument. There are three reflecting planes, D C, B C, and D. B. The angle B may be placed in such a position that the rays of light falling on the plane D B will be reflected in the direction of the arrow f; and other rays from the same distant object falling on the plane D C will be reflected in the direction of the arrow g; and the observer looking in the reverse direction of the arrows, that is, by placing his eye (or a telescope) at f, g, would see two images of the same object as if they were both at e. The angle B in this arrangement being a little more than

[graphic][subsumed][subsumed][subsumed][subsumed]

90°, affords the opportunity of making two observations of the same object at short and equal intertals, before and after the object is on the meridian. The first observation is made when the rays of light proceed in the direction of the arrows, and the second when the object is in such a position that the rays of light pursue the same path, but in the opposite direction. If both limbs of the sun or moon be observed by this arrangement of the instrument, it will furnish no less than four observations, the mean of which will give a very accurate result. In the arrangement of the planes in the subjoinod full-sized figures the angle B is rather less than 90°, which affords the opportu nity of making only a single observation. For meridional observations the instrument requires no other fixing than being placed on a level surface with the face towards the meridian sun. After being adjusted by trial, its position may be made permanent by any suitable cement.-Literary Gazette, No. 2358.

MR. BABBAGE'S CALCULATING ENGINES.

MUCH misapprehension having arisen as to the circumstances attending the invention and construction of Mr. Babbage's Calculating Engines, it is necessary to state from authority the facts relating to them.

In 1823, Mr. Babbage, who had previously invented an engine for calculating and printing tables by means of differences, undertook, at

the desire of the Government, to superintend the construction of such an Engine. He bestowed his whole time upon the subject for many years, refusing for that purpose other avocations which would have been attended with considerable pecuniary advantage. During this period, about £17,000 had been expended by the Government in the construction of the Difference Engine. A considerable part of this sum had from time to time been advanced by Mr. Babbage for the payment of the workmen, and was, of course, repaid; but it was never contemplated by either party that any portion of this sum should be appropriated to Mr. Babbage himself; and in truth, not one single shilling of the money was in any shape whatever received by Mr. Babbage for his invention, his time, or his services; a fact which Sir Robert Peel admitted in the House of Commons in March 1843.

Early in 1833, the construction of this engine was suspended on account of some dissatisfaction with the workmen. About twelve months after the progress of the Difference Engine had been thus suspended, Mr. Babbage discovered a principle of an entirely new order, the power of which over the most complicated arithmetical operations seemed nearly unbounded.

In the engine for calculating by differences, such simplifications affected only about a hundred and twenty similar parts; while in the new, or Analytical Engine, they might affect several thousands. The Difference Engine might be constructed with more or less advantage, by employing various mechanical modes for the operation of addition. The Analytical Engine could not exist without inventing for it a method of mechanical addition possessed of the utmost simplicity. In fact, it was not until upwards of twenty different modes for performing the operation of addition had been designed and drawn, that the necessary degree of simplicity required for the Analytical Engine was ultimately attained.

These new views acquired great additional importance from their bearings upon the Difference Engine already partly executed for the Government; for if such simplifications should be discovered, it might happen that the Analytical Engine would execute with greater rapidity the calculations for which the Difference Engine was intended; or that the Difference Engine would itself be superseded by a far simpler mode of construction.

Though these views might, perhaps, at that period, have appeared visionary, they have subsequently been completely realized.

To have allowed the construction of the Difference Engine to be resumed while these new views were withheld from the Government, would have been improper; yet the state of uncertainty in which those new views were then necessarily involved, rendered any written communication respecting their probable bearing on that engine a matter of very great difficulty.

From the year 1833 to the close of 1842, Mr. Babbage repeatedly applied to the Government for its decision upon the subject. These applications were unavailing. Years of delay and anxiety followed

each other, impairing those energies which were now directed to the invention of the Analytical Engine.

Amid such distractions the author of the Analytical Engine has steadily pursued his single purpose. The drawings and the notations have been freely shown; and the great principles on which the Analytical Engine is founded have been explained and discussed with some of the first philosophers of the present day. Copies of the engravings were sent to the libraries of several public institutions; and the effect of the publicity thus given to the subject is fully proved by its having enabled a distinguished Italian geometer to draw up from these sources an excellent account of that engine*.

Throughout the whole of these labours connected with the Analytical Engine, neither the Science, nor the Institutions, nor the Government of his country, have ever afforded him the slightest encouragement. When the invention was noticed in the House of Commons, one single voice † alone was raised in its favour.

During nearly the whole of a period of upwards of twenty years, Mr. Babbage had maintained, in his own house, and at his own expense, an establishment for aiding him in carrying out his views, and in making experiments, which most materially assisted in improving the Difference Engine. When that work was suspended, he still continued his own inquiries, and having discovered principles of far wider extent, he ultimately embodied them in the Analytical Engine.

The establishment necessary in the former part of this period for the actual construction of the Difference Engine, and of the extensive drawings which it demanded, as well as for the formation of those tools which were contrived to overcome the novel difficulties of the case, and in the latter part of the same period by the drawings and notations of the Analytical Engine, and the experiments relating to its constructions, gave occupation to a considerable number of workmen of the greatest skill. During the many years in which this work proceeded, the workmen were continually changing, who carried into the various workshops in which they were afterwards employed the practical knowledge acquired in the construction of these machines.

To render the drawings of the Difference Engine intelligible, Mr. Babbage had invented a compact and comprehensive language (the Mechanical Notation), by which every contemporaneous or successive movement of this machine became known. Another addition to mechanical science was subsequently made in establishing principles for the lettering of drawings; one consequence of which is, that although many parts of a machine may be projected upon any plan, it will be easily seen, by the nature of the letter attached to each working point, to which of those parts it really belongs.

* Of M. Menabrea's treatise, which appeared in the Bibliothèque Universelle de Génève for October last, a translation is given in the 12th part of the Scientific Memoirs, with copious and valuable explanatory notes by the translator. T That of Mr. Hawes, M.P. for Lambeth.

« PreviousContinue »