mystery as regards the peculiar greenish or bluish tint so long and so frequently remarked in the planetary nebulæ. It is simply consequent upon their gaseous constitution, the bright bands exhibited by their materials under intense heat having the refrangibility and the colour of those portions of the spectrum. It should, however, be observed that there is some difference among these objects in this respect; and that this is primâ facie evidence of difference of constitution. Three remarkable instances of this may be specified. One is 27 H IV, R.A. 10h. 18m. 2s. N.P.D. 107° 55′ 50′′, about 2° S. of μ Hydra, and visible in England, but catalogued by H. at the Cape as No. 3248 (Gen. Cat. 2102). Its colour is described by him as "a good sky blue," but "not so full a blue" as another, H. 3365 (Gen. Cat. 2581), which is of "a beautiful rich blue, between Prussian blue and verditter green

a feeble lamp light gives it a deep indigo contrasted colour. Ditto if a red star, n p, about 10' distant, be brought into the field with it." The 3rd, on the other hand, H. 3594 (Gen. Cat. 4066), "a most elegant and delicate planetary nebula," equal to a star 8.9 mag., and "perfectly sharp, not the slightest haziness," is specified as having "a clear round planetary white disc." It is much to be regretted that these last two are out of the reach of the English spectroscope, and could not even be touched at the Roman observatory, where, we were told, a twelvemonth ago, that O. Struve was astonished at the magnificence of the stellar spectra, though he had previously seen those at Greenwich with a much larger aperture. Merz has since been at work for the same observatory, upon a yet finer spectroscope, with 2 prisms of 60°, and a dispersion double that of the ordinary Frauenhofer glass, producing a total deviation of more than 120°.

Some further details as to nebular researches must be postponed.

OCCULTATIONS.

April 9th, 8 Aquarii, 6 mag., 15h. 22m. to 16h. 16m.18th, 111 Tauri, 6 mag., 10h. 57m. to 11h. 40m.-29th, μ Libra, 6 mag., 15h. 47m. to 16h. 15m.

ON THE APPLICATION OF RECENT SCIENTIFIC DISCOVERIES TO THE USEFUL ARTS.

BY PROFESSOR M'GAULEY.

No subject can be more interesting than that of the aid which scientific research has afforded to human progress. The career of mighty conquerors has been marked by devastation and blood. Populous cities have been changed by them into heaps of ruins; rich and highly-cultivated regions into "howling wildernesses." How different have been the results of peaceful research, the only object of which has been the benefit of mankind. If the perusal of history, as the record of splendid victories and terrible defeats, of oppression and the struggles of the oppressed, is much more calculated to produce feelings of pain than of pleasure, the consideration of those wonderful improvements in our condition which have been effected by men who are often as deeply absorbed in their ennobling studies, and as unconscious of the turmoil of the world around them as was Archimedes of the storming of Syracuse, are suited to call forth feelings of unalloyed pleasure.

The triumphs of genius and industry over the difficulties with which the path of progress is beset, meet us wherever we turn. There are periods at which our advance has been exceptionally rapid; one of the most remarkable of these is comprehended within the present century, and chiefly within the more recent portions of it. The differences between the advantages which we enjoy, and for which we are indebted to science, and those possessed by our immediate predecessors, is scarcely less great than that between the condition of the polished Romans and the Britons whom they subdued.

An acquaintance with what has been already done, and with the difficulties which others have surmounted by ingenuity and peseverance, creates a powerful tendency to further exertion. The records of science abound with examples of sudden and rapid strides, on account of the attention of mankind being turned to some principle, or some fact, which perhaps had been long known, but as long neglected. The properties of steam have, no doubt, at all times, attracted notice but they were never usefully applied, until the Marquis of Worcester, Savory, Newcomen, and Watt rendered them, by their researches, the subject of deep and general interest. Good service is therefore done, when the public mind is strongly directed to what science has already achieved. But much as the experimentalist has done, there is no reason to

suppose that a thousand times as much will not hereafter be accomplished. There is not one who, if he try, may not strike on some vein of useful discovery, that shall place his name in the list of the benefactors of his race.

It would, of course, be impossible, within the limits of one or two papers, however long they might be made, to touch on even a small portion of the applications of science to useful purposes. We shall, therefore, be obliged to content ourselves with the selection of a few examples from among those which appear best to illustrate the great advantages that have been conferred by the thinker and the experimentalist on arts and manufactures.

Among the contrivances which benefit mankind, none are of greater utility than those which effect a saving of labour. In former times, the production of works at all remarkable for their vastness, was the result of a painful struggle between brute force and the laws of nature. Refractory materials were rent asunder, and the fragments carried to where they were require, only by the most exhausting exertions. To build the great pyramid required the labour of 100,000 men for twenty years, and during the work a very large number of them perished miserably through excessive toil. And yet, how insignificant are all the pyramids, taken together, in comparison with the mighty works constructed with ease, and without the loss of a single life through over exertion, by the modern engineer, on even even one railway; thanks to men of science who have furnished him with those appliances which have caused every difficulty to vanish. Mere physical labour has, in a great degree, been transferred to automatic machines, that require little more than a superintendence, which, being an operation of the mind, is calculated not to degrade but to elevate.

The use of atmospheric air for the transmission of force. is among the simplest applications of science to industrial purposes. We shall glance at a few of the methods which have been employed for the purpose. The wind has indeed long been used as a source of motion; but its uncertainty and varying force have rendered the effect which may be obtained from it of but little value. The air is far more usefully applied to the storeing up, or the transmission of power, which may be used when and where it might not have been convenient or even possible to obtain it from other sources. Its efficiency may be derived from either increased or diminished pressure.

The possibility of impelling carriages on railways by means of atmosphere pressure, suggested itself long since; but the difficulties with which the carrying out of the project was beset,

and the wrong principles that were adopted in the attempt to do so, rendered, until recently, all experiments on the subject unsuccessful. The very earliest idea was that the carriages should travel within a tube; but it was abandoned, from a supposition that it would be impossible to remain with comfort, or even with safety, in such circumstances. A piston, moving inside the tube, and connected by means of a long valve with carriages external to it, was therefore adopted. But no amount of ingenuity or care was found sufficient to prevent a leakage that caused a loss of power incompatible with the profitable working of such a line. The leakage was the more serious from the considerable difference between the pressure required at each side of the piston. It has been found that when the carriages or waggons are within the tube, a pressure of three or four ounces per square inch will suffice for all desirable velocities-two and a half ounces are enough to produce a speed of thirty or forty miles an hour. But when there is a travelling piston, and a valve the whole length of the tube, to allow the bar connecting the piston and carriages to pass along it, from 120 to 160 ounces are indispensable.

The history of the atmospheric railway furnishes an example of the abandonment of a valuable principle, simply because unscientific means were used for carrying it out, and difficulties which had no existence were taken for granted. In the infancy of the locomotive, a similarly unfounded assumption of insufficient adhesion between the driving wheels and the rails led to a great waste of ingenuity, and to a large and useless expenditure of time and money.

The application of atmospheric pressure to the propulsion of vehicles might have long remained in abeyance, had not the Pneumatic Despatch Company applied the principle to the conveyance of letters and merchandize, and thus afforded an opportunity of proving that even human beings might continue without inconvenience in a closed tube. The removal of an apparently insuperable objection to atmospheric propulsion which has thus taken place is of the highest importance; since, with a pneumatic railway, the danger of accident is diminished almost to nothing; collisions are made absolutely impossible; and since a train cannot, in any circumstances, run off the line, the greatest velocity is unaccompanied by danger. The view of external objects is, indeed, altogether excluded; but this privation is little greater than that experienced on ordinary railways from the number of cuttings, tunnels, and interposed objects, and the high speed.

With an atmospheric railway, the expense of construction and maintenance are greatly diminished; steep gradients and sharp curves cease to be objectionable, since an ascent of one

in fifteen, or a curve of eight chains radius, causes no inconvenience, and great facilities are afforded by it for passing under rivers.

The elasticity of the air has been applied in numberless other ways. Reducing its pressure during evaporation had led to extraordinary economy in the manufacture of cane sugar; and to the removal of all disagreeable smell and danger of explosion from petroleum. Bridge building has been greatly facilitated by exhausting the air within the caissons sunk for the foundation of piers, which causes the silt and mud to rise up within them, so as to be easily removed; or by keeping the water out of the caisson, while the excavation is being carried on, the air within it being compressed. The process used in the manufacture of soda has been simplified by discovery of the fact that sulphate of baryta is decomposed by caustic lime, at a pressure greater than that of the atmosphere.

Compressed air is applied very effectively to the transmission of power. Thus, in giving motion to the shuttle in weaving, and on the large scale in the tunnel which is being cut under the Alps near Mont Cenis. The air, in the latter case, is condensed by water power outside, and is conveyed by pipes into the excavation, where it gives motion to the chisels that perforate the rock, forming cavities for the gunpowder used in blasting.

The contrivances employed as convenient substitutes for human labour are not set in motion by air only; water pressure also is employed with great advantage for the purpose. The water-wheel, like the windmill, is of ancient date, and, like it, is unsuited in ordinary cases to the production of motion. The pressure of water has therefore been otherwise and more effectively applied-very often by means of an engine bearing a very close resemblance to the steam engine, and differing from it only in consequence of the difference between the properties of non-elastic and elastic fluids: a difference which was long a source of great difficulties. A water pressure of between sixty and seventy pounds to the square inch is found sufficient for all ordinary purposes, and may be obtained from the water supply of most great towns and cities. Water, on account of its freedom from danger, and the facility with which its pressure is transmitted from one place to another by means of pipes, as well as its economy and easy application, will most probably before long come into very general use.

The discovery of means for propelling machinery by air or water pressure has been the cause of great improvements in mining. The idea of substituting mechanical for hand labour