XCVI. An Attempt to Improve the Manner of Working the Ventilators by the Help of the Fire-Engine By Keane Fitzgerald, Esq. F.R.S. p. 727. Dr. Hales wishing to extend his useful ventilators to those who work in mines at great depths under ground, where the lives of many are lost by noxious vapours, cccasioned by the want of a free circulation of air; and finding by experience, that ventilators worked by wind do not operate above one third part of the year, and in calm hot weather, when most wanted, do not operate at all; he applied to Mr. Fitzgerald for assistance in contriving a machine to work the ventilator by the help of the fire-engine, which is now generally used in all mines for drawing off the water: and which he accordingly attempted, and hoped it would answer the purpose.

As the lever of the fire-engine works up and down alternately, and performs at a common medium about a dozen strokes in a minute, it was necessary to contrive some way to make the beam, though moving alternately, to turn a wheel constantly round one way, and also to increase the number of strokes to 50 or 60 in a minute. The model of a machine for this purpose is composed of 4 wheels of different sizes, 2 clicks, 3 pinions, and a fly; which is put into motion by the part of a wheel fixed to the arch of a lever of the fire-engine. The wheel which is turned by the lever, or rather moved up and down by it, is loose on its arbor; and likewise one of the rochets, and the wheel next to it. The outside rochet and outside wheel are fixed on the arbor. There are two pinion-wheels fixed on the arbor; one on each side, near the edge of the wheel moved by the lever which turns them. There are also two clicks; one fixed to the great wheel, the other to the frame. These exclusive of the wheel that moves the fly.

The effect is, when the lever moves the wheel downwards, its click forces the rochet fixed on the arbor to move along with it, and the other wheels the same way. When it moves upwards the click fixed on the frame stops the larger rochet, and the wheel next to it, which are pinned together. This wheel being stopped, and the great wheel carried upwards by the lever, the pinion towards the edge of the great wheel is forced round it, and moves the pinion on the other side the great wheel; which pinion moves the wheel fixed on the arbor, the contrary way to the great wheel, which is carried upwards by the lever. By which means the arbor is constantly turned the same way, when the lever of the fire-engine is moved either upwards or downwards.

Upon the arbor there is also another great wheel fixed, which turns a pinion: on the arbor of which pinion is a crank to move the ventilator, and also a fly fixed to the end, to help the motion of the crank, which in the model is turned 3 times for each stroke of the lever, and may be increased or diminished, ac

cording to the number of teeth in the pinion. The number of teeth in the great wheel moved by the lever is 66; but need not have teeth above half way round. The wheel fixed to the rochet has 33 teeth, and its pinion 11. The wheel fixed on the arbor on the outside, has 24 teeth, and its pinion 16. The wheel which turns the fly has 90 teeth, and the pinion turned by this wheel 10. The greater the number of teeth in the rochets, the better. This machine may also be applied to other useful purposes at mines; and it may be easily made to turn a mill to grind corn; or to turn a wheel to raise coals, or whatever else is wanted to be raised from the mines.

XCVIII. Of some Experiments concerning the Different Refrangibility of Light. By Mr. John Dollond. With a Letter from James Short, M. A., F.R.S. p. 733.

Mr. Short's introductory letter is as follows:

I have received the inclosed paper from Mr. Dollond, which he desires may be laid before the Royal Society. It contains the theory of correcting the errors arising from the different refrangibility of the rays of light in the object-glasses of refracting telescopes; and I have found on examination, that telescopes made according to this theory are entirely free from colours, and are as distinct as reflecting telescopes. The following by Mr. Dollond:

It is well known, that a ray of light, refracted by passing through mediums of different densities, is at the same time proportionally divided or spread into a number of parts, commonly called homogeneal rays, each of a different colour; and that these after refraction, proceed diverging: a proof that they are differently refracted, and that light consists of parts that differ in degrees of refrangibility. Every ray of light passing from a rarer into a denser medium, is refracted towards the perpendicular; but from a denser into a rarer one, from the perpendicular; and the sines of the angles of incidence and refraction are in a given ratio. But light consisting of parts which are differently refrangible, each part of an original or compound ray has a ratio peculiar to itself; and therefore the more a heterogene ray is refracted, the more will the colours diverge, since the ratios of the sines of the homogene rays are constant; and equal refractions produce cqual divergencies. That this is the case when light is refracted by one given medium only, as suppose any particular sort of glass, is out of all dispute, being indeed self-evident; but that the divergency of the colours will be the same under equal refractions, whatever mediums the light may be refracted by, though generally supposed, does not appear quite so clearly.

However, as no medium is known which will refract light without diverging the colours, and as difference of refrangibility seems thence to be a property inherent in light itself, opticians have on that consideration, concluded, that equal

refractions must produce equal divergencies in every sort of medium; whence it should also follow, that equal and contrary refractions must not only destroy each other, but that the divergency of the colours from one refraction would likewise be corrected by the other; and there could be no possibility of producing any such thing as refraction, which would not be affected by the different refrangibility of light; or, in other words, that however a ray of light might be refracted backward and forward by different mediums, as water, glass, &c. provided it was so done that the emergent ray should be parallel to the incident one, it would ever after be white; and conversely, if it should come out inclined to the incident, it would diverge, and ever after be coloured. From which it was natural to infer, that all spherical object-glasses of telescopes must be equally affected by the different refrangibility of light, in proportion to their apertures, whatever material they may be formed of.

But it seems worthy of consideration, that notwithstanding this notion has been generally adopted as an incontestable truth, yet it does not seem to have ́ been hitherto so confirmed by evident experiment, as the nature of so important a matter justly demands; and this it was that determined Mr. D. to attempt putting the thing to issue by the following experiment. He cemented together two plates of parallel glass at their edges, so as to form a prismatic or wedgelike vessel, when stopped at the ends or bases; and its edge being turned downward he placed therein a glass prism with one of its edges upward, and filled up the vacancy with clear water; thus the refraction of the prism was contrived to be contrary to that of the water, so that a ray of light transmitted through both these refracting mediums would be refracted by the difference only between the two refractions. Therefore, as he found the water to refract more or less than the glass prism, he diminished or increased the angle between the glass plates, till he found the two contrary refractions to be equal; which he discovered by viewing an object through this double prism; which, when it appeared neither raised nor depressed, he was satisfied that the refractions were equal, and that the emergent rays were parallel to the incident.

Now, according to the prevailing opinion, the object should have appeared. through this double prism quite of its natural colour; for if the difference of refrangibility had been equal in the two equal refractions, they would have rectified each other but the experiment fully proved the fallacy of this received. opinion, by showing the divergency of the light by the prism to be almost double of that by the water; for the object, though not at all refracted, was yet as much infected with prismatic colours, as if it had been seen through a glass wedge only, whose refracting angle was near 30 degrees. This experiment will be readily perceived to be the same as that which Sir Isaac Newton mentions, book 1, part 2, prop. 3, experiment 8, of his Optics. But how it comes

to differ so very remarkably in the result, Mr. D. does not take upon him to account for; but only adds, that he used all possible precaution and care in the process, and that he kept the apparatus by him to evince the truth of what he wrote, whenever he might be properly required so to do.

He plainly saw then, that if the refracting angle of the water-vessel could have admitted of a sufficient increase, the divergency of the coloured rays would have been greatly diminished, or entirely rectified; and there would have been a very great refraction without colour, as now he had a great discolouring without refraction but the inconveniency of so large an angle, as that of the vessel must have been to bring the light to an equal divergency with that of the glass prism, whose angle was about 60 degrees, made it necessary to try some experiments of the same kind by smaller angles.

He ground a wedge of common plate glass to an angle of somewhat less than 9 degrees, which refracted the mean rays about 5 degrees. He then made a wedge-like vessel, as in the former experiment, and filling it with water, managed it so that it refracted equally with the glass wedge; or in other words, the difference of their refractions was nothing, and objects viewed through them appeared neither raised nor depressed. This was done with an intent to observe the same thing over again in these small angles which he had seen in the prism: and it appeared indeed the same in proportion, or as near as could be judged; for notwithstanding the refractions were here also equal, yet the divergency of the colours by the glass was vastly greater than that by the water; for objects seen by these two refractions were very much discoloured. Now this was a demonstration, that the divergency of the light, by the different refrangibility, was far from being equal in these two refractions. He also saw, from the position of the colours, that the excess of divergency was in the glass; so that he increased the angle of the water-wedge by different trials, till the divergency of the light by the water was equal to that by the glass; that is, till the object, though con-siderably refracted by the excess of the refraction of the water, appeared nevertheless quite free from any colours proceeding from the different refrangibility of light; and, as near as he could then measure, the refraction by the water was about of that by the glass. Indeed he was not very exact in taking the measures, because the business was not at that time about the proportions, so much as to show that the divergency of the colours, by different substances, was by no means in proportion to the refractions; and that there was a possibility of refraction without any divergency of the light at all.

Having, about the beginning of the year 1757, tried these experiments, he soon after set about grinding telescopic object-glasses on the new principles of refractions, which he had gathered from them; which object-glasses were compounded of two spherical glasses with water between them. These glasses he

had the satisfaction to find, as he had expected, free from the errors arising from the different refrangibility of light: for the refractions by which the rays were: brought to a focus, were every where the differences between two contrary refractions, in the same manner, and in the same proportions as in the experiment with the wedges.

very

However, the images formed at the foci of these object-glasses, were still far from being so distinct as might have been expected from the removal of so great a disturbance; and yet it was not very difficult to guess at the reason, when it is considered, that the radii of the spherical surfaces of those glasseswere required to be so short, in order to make the refractions in the required proportions, that they must produce aberrations or errors in the image, as great, or greater than those from the different refrangibility of light. And therefore, seeing no method of getting over that difficulty, he gave up all hopes of succeeding in that way. And yet, as these experiments clearly proved, that different substances diverged the light very differently in proportion to the refraction; he began to suspect that such variety might possibly be found in different sorts of glass, especially as experience had already showed that some made much better object-glasses in the usual way than others: and as no satisfactory cause had as yet been assigned for such difference, there was great reason to presume, that it might be owing to the different divergency of the light by their refractions.

Therefore the next business to be undertaken, was to grind wedges of different kinds of glass, and apply them together, so that the refractions might be made in contrary directions, in order to discover, as in the foregoing experiments, whether the refraction and divergency of the colours would vanish together. But a considerable time elapsed before he could set about that work; for though he was determined to try it at his leisure, for satisfying his own curiosity, yet he did not expect to meet with a difference sufficient to give room for any great improvement of telescopes; so that it was not till the latter end of the year that he undertook it, when his first trials convinced him that this business really deserved his utmost attention and application.

He discovered a difference far beyond his hopes, in the refractive qualities of different kinds of glass, with respect to their divergency of colours. The yellow or straw-coloured foreign sorts, commonly called Venice glass, and the English crown glass, are very near alike in that respect, though in general the crown glass seems to diverge the light rather the less of the two. The common plate glass made in England diverges more; and the white crystal or fi:nt English glass, as it is called, most of all. It was not now his business to examine into the particular qualities of every kind of glass that he could come at, much less to amuse himself with conjectures about the cause, but to fix on such two sorts as their difference was the greatest; which he soon found to be the crown, and,