Owing to the oblique position of the axis, it was impossible to see the comet in the inferior part of its orbit E V, on its approach to the perihelion. XVI. A Second Letter from E. V. on the Means of gaining Power in Mechanics. SIR, To Mr. Tilloch. MUCH as the moderns are reckoned to surpass the antients in mathematical knowledge, and notwithstanding the experience and improvements in mechanics, during the later and more enlightened ages, it is a mortifying truth that we are even at this day totally ignorant of the means formerly employed, and very extensively in use, to move to vast distances, and raise to great height, prodigious masses and weights, such as the celebrated columns of Egypt, Rome, &c.; and that with all our advantages over the antients, we still remain unable to equal their practice in these respects. It does not appear that our predecessors were gifted with superior intellect or strength to the present race; their means must have been mechanical, and therefore must be within our reach.-Why then should we not attempt and expect to do as much as they did? It is truly surprising that an art of so useful and important a nature, so nearly allied to the mechanical powers in constant varied use and progressive improvement among us, has so long been lost and escaped discovery! Perhaps the time is not distant when this mighty secret shall again be common for the general benefit of society. My humble endeavours have occasionally been directed to this end; and with this view I have contributed my mite, and shall continue so to do by the communication, through your valuable Magazine, of relative experiments and results which may be novel, in the hope of stimulating the exertions of the more scientific ;-for, as "great evenis oft owe their rise to trivial cause," my hints may baply furnish abler heads with ideas which may lead further than I can pretend to penetrate; and thus, between us, the period of success may be accelerated. In my last, I gave account of an engine of my contrivance, which gained considerable power :-this engine has since been so improved, that with the same moving power, the same velocity, &c., it raises a weight of thirty pounds instead eve that it may yet be made far more powerful. Aware of the reception which this extraordinary fact, so ontrary to established opinions, is likely to meet, I have ought, and been so fortunate as to find, additional support n further experiments, and the construction of anotherngine materially different from the first, which, however, roduces similar, and indeed greater effects. More of this shall be the subject of my next paper; and n the mean time, those of your numerous readers, who nay think with me, that the means of gaining power cannot be limited to the two which I have thus announced, have a wide field opened for the exercise of their genius in this reI have the honour to be, sir, search. Bracknell, Feb. 23, 1808. your very humble servant, E. V. XVII. On the Use of Sulphur as a Vermifuge. By JOSEPH HUME, Esq., of Long-Acre, London. SIR, To Mr. Tilloch. HAVING been favoured with some additional information respecting the efficacy of sulphur, and the proper way of applying it to vegetables, I now fulfil the conditional promise I made in my last letter. The method is truly simple; for nothing more is required than to sprinkle sublimed sulphur, or, as it is commonly called, flowers of brimstone, over the leaves of the tree or plant, wherever the effects of worms or insects prevail, or may be expected to come. This may be so easily accomplished, that it seems superfluous to point out any particular plan or apparatus. The sulphur may be tied up in a piece of muslin or linen, and with this the leaves and young shoots should be dusted; or it may be thrown on by means of a common swandown puff, or even a dredging-box. However, if this practice become general, no doubt some convenient instrument, Phil. Mag. vol. xxix. p, 959. probably probably a pair of bellows, constructed on purpose, will be contrived, so as to prevent unnecessary loss of the sulphur. By the same friendly communication I have received fresh assurances, not only of the powerful influence of sulphur as a vermifuge against the whole tribe of wornis and other insects, which infest, and prey upon, vegetables; but I also find, that in other respects this substance is even congenial to the health of those trees and plants on which it is sprinkled that peach-trees in particular were remarkably improved by the sulphur; they absorbed it, and, it may be said, were even fond of it; for it was evidently absorbed, and must have entered into the vegetable system. It was likewise noticed, that the verdure and other healthful appearances were perceptibly increased; for the quantity of new shoots and leaves formed subsequently to the operation, and having no suiphur on their surfaces, served as a kind of comparative index, and pointed out distinctly the accumulation of health. Upon the whole, it may be observed, that, independently of its deleterious effects on the vermin, the question respecting these sanative powers of such an insoluble substance as sulphur, seems to be one of the utmost importance; and, I should think, must be highly interesting to the physiologist, and, indeed, to all men of science. I remain, sir, Long-Acre, Feb. 24, 1808. your obliged and obedient servant, XVIII. Experiments for investigating the Cause of the coloured concentric Rings, discovered by Sir ISAAC NEWTON, between two Object-glasses laid upon one another. By WILLIAM HERSCHEL, LLD. F.R.S.* THE account given by Sir I. Newton, of the coloured arcs and rings which he discovered by laying two prisms or ob * From Philosophical Transactions for 1897, Part II. ject ject-glasses upon each other, is highly interesting. He very justly remarks, that these phænomena are " of difficult consideration," but that "they may conduce to further discoveries for completing the theory of light, especially as to the constitution of the parts of natural bodies on which their colours or transparency depend *." With regard to the explanation of the appearance of these coloured rings, which is given by Sir I. Newton, I must confess that it has never been satisfactory to me. He accounts for the production of the rings, by ascribing to the rays of light certain fits of easy reflection and easy transmission alternately returning and taking place with each ray at certain stated intervals t. But this, without mentioning particular objections, seems to be an hypothesis which cannot be easily reconciled with the minuteness and extreme velocity of the particles of which these rays, according to the Newtonian theory, are composed. The great beauty of the coloured rings, and the pleasing appearances arising from the different degrees of pressure of the two surfaces of the glasses against each other when they are formed, and especially the importance of the subject, have often excited my desire of inquiring further into the cause of such interesting phænomena; and with a view to examine them properly I obtained, in the year 1792, the two object-glasses of Huygens, in the possession of the Royal Society, one of 122 the other of 170 feet focal length, and began a series of experiments with them, which, though many times interrupted by astronomical pursuits, has often been taken up again, and has lately been carried to a very considerable extent. The conclusions that may be drawn from them, though they may not perfectly account for all the phænomena of the rings, are yet sufficiently well supported, and of such a nature as to point out several modifications of light that have been totally overlooked, and others that have never been properly discriminated. It will, therefore, be the aim of this paper to arrange and distinguish the yarious, modifications of light in a clear and perspicuous Newton's Optics, 4th ed. p. 169, † Ibid. p. 256. order, |