applicable to machinery. This consists in employing a voltaic battery, generally of trifling power, by means of which magnetism may be generated in a bar of soft iron, by simply wrapping it with a lengthened coil of copper wire, previously covered with an envelope of silk or cotton thread, and then connecting the ends of the wire, one with the copper, and the other with the zinc element of the battery. The bar is thus at once converted into an electro-magnet, and possesses, so long as the communication continues, all the properties of a permanent one, the power generated increasing in proportion to the quantity of wire employed. With 60 feet of wire, for example, Professor Henry caused such a bar as we have described to raise 7 lb. avoirdupois, but when the wire was increased to eight times the length, he found it to have acquired the amazing lifting power of 650 lbs.

But that which renders the electro-magnet likely to become valuable as a moving power, depends chiefly on another principle. Unlike the permanent magnet, the polarity of its two ends results entirely from the connection which the ends of the wire are made respectively to have with the copper or zinc of the battery; so that, as often as this communication is interrupted, the bar loses most of its magnetic properties, and when the connection is reversed, a corresponding change takes place in the poles, that becoming instantly north which formerly was south, and vice versa.

The mode of generating motion by such an agent will easily be understood by supposing a case. Let a horseshoe electro-magnet be fixed in a perpendicular position, with its poles pointing upwards, and let there be suspended over them a bar magnet. This bar will, of course, immediately arrange itself according to the common rule of magnetism, its north and south poles seeking respectively the south and north poles of the electro-magnet. Next, let the ends of the coil of wire be shifted, so that the end which communicated with the copper may now be connected with the zinc, and that attached to the zinc

may now communicate with the copper. The electromagnet immediately reverses the polarity of its ends,the north pole acquiring south polarity, and the south pole becoming converted into the north. The suspended magnet turns quickly round, under the influence of the new attractions and repulsions, and if the process be repeated, it acquires a rotary motion, more or less rapid, according to the rapidity with which these changes are made. It will be unnecessary to pursue this subject farther, as might be done, by showing to what an extent power may be rendered available, by the use of a number of electro-magnets at once; and how the nature of the discovery may be enhanced by employing electromagnetism as well in the revolving as in the stationary parts of the apparatus. The intelligent reader will easily fill up the blank for himself, and will readily comprehend how the principle thus manifested may be applied to the purposes of machinery.

The ingenuity of practical men has already begun to apply it under a vast variety of modifications, some of which have been proved to be perfectly available for the most important purposes of mechanism. Mr M'Gauly, in Ireland, and Mr Davenport and Dr Page, in the United States, seem to have been hitherto the most zealous and most successful adventurers in this important field,the last mentioned gentleman having this year produced an instrument capable, as he affirms, of generating a rate of revolution equal to six thousand in a minute. It would appear that, up to a very recent date, the only electro-magnetic machine actually in use, was one in the occupation of Mr Sturgeon of Woolwich, employed for pumping water and other mechanical purposes.

To what a train of reflections does this discovery give rise, and what an exposure does it make of the ignorance of man, even amid the boasted progress of science and improvement! For how many hundred years have we been acquainted with the properties of the magnet! How long have we been habituated to the phenomena of

the voltaic battery; and yet, with the elements of a power so extraordinary daily in our hands, we have failed, till this late hour, in effecting this important and valuable invention, which, though among the simplest, is likely to be among the most useful, of the modern applications of the physical properties of matter. Then to what an unfathomable depth of future discoveries does this seem to open the avenue! Who can tell what valuable treasures lie hid among the simple elements with which we are familiar, or how amply these may yet be made to co-operate with man in the coming ages of his history! G. J. C. D.

TWELFTH WEEK-TUESDAY.

ARCHITECTURE.-ITS MODERN HISTORY AND PRACTICE-LIGHTHOUSES THE EDDYSTONE LIGHTHOUSE.

AMONG buildings of public utility, lighthouses take, in a commercial country, a prominent station. These edifices, however, have diminished, both in number and utility, since the use of the mariner's compass; and, instead of being placed on every headland, as was probably the case in the earlier ages of navigation, where trade was carried on to any extent, as in the Mediterranean, they are now chiefly confined to the marking of dangerous rocks at sea, and the most projecting points of the shore. The most remarkable lighthouse of antiquity was that of the Pharos of Alexandria, which was accounted one of the seven wonders of the ancient world. It is said to have been erected by Ptolemy Philadelphus, about 300 years before the Christian era, and is alleged, probably with some exaggeration, to have been a hundred statures of a man, or about five hundred and fifty feet, in height. It is described as a square building of exquisite workmanship. On the top, a fire was kept

constantly burning, which some authors assert could be seen at the distance of a hundred, and others of even seven hundred miles ;* but which Josephus more soberly and truly limits to the distance of three hundred stadia, or about forty-two British miles. Even at this distance, on account of the convexity of the earth's surface, it could only be seen by a spectator elevated upwards of a hundred feet above the sea.

The utility of lighthouses to the mariner is too great and too obvious to suffer them to be neglected in modern times. The object of the present paper, however, is not to give even the slightest sketch of the history of these useful works, but to lay before the reader some account of what may be ranked among the most arduous and successful architectural operations of modern times, in the erection of successive lighthouses on the Eddystone reef.†

This extensive reef is situated in the English Channel, at the entrance of Plymouth Sound, and is dangerous to mariners, and particularly disadvantageous on account of its position as regards the celebrated naval station in its immediate neighbourhood. It had, at an early period therefore, been a matter of anxious consideration to effect the erection of a beacon on a rock of this reef; but there were circumstances which rendered such an undertaking peculiarly hazardous. From the nature of its position, it lies open to the remarkable swells of the Bay of Biscay and the Atlantic Ocean. "It is to be observed," says Mr Smeaton, in his highly interesting account of his own operations, "that the soundings of the

* It is said, very absurdly, by Abulfeda, that this light, when increased by the use of a speculum, could be seen at Constantinople, which is 700 miles distant from Alexandria !

+ I am indebted for an opportunity of giving so detailed a history of the celebrated lighthouses erected on this remarkable spot, to my friend, Mr Stevenson of Edinburgh, who was so good as to put into my hands the account published by Mr Smeaton, the ingenious architect of the last erection on the Eddystone reef, a kindred genius, whose labours he has imitated and rivalled on the Bell Rock. As Mr Smeaton's work is exceedingly scarce, and, because of its minute scientific details, is scarcely suitable to the general reader, I persuade myself that the account I have here given will be acceptable.

sea from the south-west towards the Eddystone, are from eighty fathoms to forty, and every where, till you come near the Eddystone, the sea is full thirty fathoms in depth; so that all the heavy seas from the south-west, come uncontrolled on the Eddystone rocks, and break thereon with the utmost fury." Mr Smeaton further remarks, that, on account of the configuration of the rocks, which lie in a sloping manner towards the southwest, the situation is still more exposed to the violence of the waves; for as they come unchecked from the deep water, and meet the slope of the rocky bottom "rather suddenly at last, though gradually," they are swelled to such a degree in storms and hard gales of wind, as to break upon the rocks with greatly augmented fury. In moderate, and even in calm weather, the effect of this peculiarity is strongly felt; "for the libration of the water caused in the Bay of Biscay in hard gales at south-west, continues in these deep waters for many days, though succeeded by a calm; insomuch, that when the sea is, to all appearance, smooth and even, and its surface unruffled by the slightest breeze, yet those librations still continuing, which are called the ground swell, and meeting the slope of these rocks, the sea breaks upon them in a frightful manner, so as not only to obstruct any work being done upon the rock, but even the landing upon it, when, figuratively speaking, you might go to sea in a walnut-shell." Add to this, that the rock, which alone is fit for the erection of a lighthouse, presents towards the west so abrupt a face, that the seas, when swelling upon it even in moderate weather, meet a sudden check, so that they frequently fly to the height of thirty or forty feet," falling down in copious showers on the surface of the rock.

These circumstances rendered the erection of any kind of building on the Eddystone a work of peculiar difficulty, insomuch, that it was deemed altogether impracticable, till a person of the name of Winstanley, a man of eccentric genius, undertook, and, in the year