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and destruction far beyond the reach of any historical knowledge. In Cæsar's time, the level of the North Sea appears to have been the same as in our days. He mentions the separation of the Wahal branch of the Rhine, and its junction to the Meuse; noticing the then existing distance from that junction to the sea; which agrees, according to D'Anville's enquiries, with the actual distance. Some of the Roman roads constructed by order of Augustus, under Agrippa's administration, leading to the maritime towns of Belgium, still exist, and reach the present shore. It seems proved, from historical records, carefully collected by several learned members of the Brussels Academy, that no material change has happened to the lowermost part of maritime Flanders during the period of the last 2000 years.

Investigation of the Powers of the Prismatic Colours to heat and illuminate Objects. By WILLIAM HERSCHEL, LL.D. Experiment 1. Having arranged two thermometers in the place prepared for the experiment, I waited till they were stationary. Then, advancing No. 1. to the red rays, and leaving the other two close by, in the shade, I marked down what they showed at different times, as annexed. This, in about eight or ten minutes, gave 63 degrees, for the rising produced in my thermometer, by the red rays, compared to the standard thermometer.

Experiment 2.-Proceeding in the same manner as before, in the green rays I had as annexed. Therefore, in ten minutes, the green rays occasioned a rise of 34 degrees.

Experiment 3. I now exposed my thermometer to the violet rays, and compared it with No. 2. Here we have a rising of 2°, in ten minutes, for the violet rays.

No. 1 No. 2.

49

50

No. 1.

43
48

43

43

43

493

431

43

No. 2.

43

43

45

43

46

43

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From these experiments, we are authorised to draw the following results. In the red rays, my thermometer gave 6 degrees for the rising of the quicksilver. In the second experiment, we had 34 degrees, for the rising occasioned by the green rays; from which we obtain the proportion of 55 to 26, for the power of heating in red to that in green. The third experiment gave 2° for the violet rays; and

therefore we have the rising of the quicksilver in red to that in violet, as 55 to 16. Therefore, we have the proportion of the rising in red to that in green, as 27 to 11, or as 55 to 22.4.

Experiment on the illuminating Power of coloured Rays. -I placed an object that had very minute parts under a double microscope; and having set a prism in the window, so as to make the coloured image of the sun stationary on the table where the microscope was placed, I caused the differently coloured rays to fall successively on the object, by advancing the microscope into their light. The magnifying power was 27 times.

By an attentive and repeated inspection, I found that my object was very well seen in red; better in orange, and still better in yellow; full as well in green; but to less advantage in blue; indifferently well in indigo, and with more imperfection in violet.

From these and other observations we may conclude, that the red-making rays are very far from having it in any eminent degree. The orange possesses more of it than the red; and the yellow rays illuminate objects still more perfectly. The maximum of illumination lies in the brightest yellow, or palest green. The green itself is nearly equally bright with the yellow; but, from the full deep green, the illuminating power decreases very sensibly. That of the blue is nearly on a par with that of the red: the indigo has much less than the blue; and the violet is very deficient.

As an easy way of smoking glasses uniformly is of some consequence in astronomical observations, it may be of service here to give the proper directions, how to proceed in the operation.

With a pair of warm pliers, take hold of the glass, and place it over a candle, at a sufficient distance not to contract smoke. When it is heated, but no more than still to permit a finger to touch the edges of it, bring down the glass, at the side of the flame, as low as the wick will permit, which must not be touched. Then, with a quick vibratory motion, agitate it in the flame from side to side; at the same time advancing and retiring it gently all the while. By this method, you may proceed to lay on smoke to any required darkness. It ought to be viewed from time to time, not only to see whether it be sufficiently dark, but whether any inequality may be perceived; for if that should happen, it will not be proper to The smoke of sealing-wax is bad: that of pitch is A wax candle gives a good smoke; but that of a

go on. worse.

tallow candle is better. As good as any I have hitherto met with, is the smoke of spermaceti oil. In using a lamp, you may also have the advantage of an even flame extended to any length.

Experiments on the Refrangibility of the invisible Rays of the Sun. By WM. HERSCHEL, LL.D. F. R. S. — [ 1800.]

My experiments prove, that there are rays coming from the sun, which are less refrangible than any of those that affect the sight. They are invested with a high power of heating bodies, but with none of illuminating objects; and this explains the reason why they have hitherto escaped unnoticed. At the distance of 52 inches from the prism, there was still a considerable heating power exerted by invisible rays, 11⁄2 inch beyond the red ones, measured on their projection on a horizontal plane. I have no doubt but that their efficacy may be traced still somewhat farther. Experiments show, that the power of heating is extended to the utmost limits of the visible violet rays, but not beyond them; and that it is gradually impaired, as the rays get more refrangible. The maximum of the heating power is vested among the invisible rays; and is, probably, not less than half an inch beyond the last visible ones. The same experiments alsoshow, that the sun's invisible rays, in their less refrangible state, and considerably beyond the maximum, still exert a heating power fully equal to that of red-coloured light; and that, consequently, if we may infer the quantity of the efficient from the effect produced, the invisible rays of the sun probably far exceed the visible ones in number.

If we call light, those rays which illuminate objects, and radiant heat, those which heat bodies, it may be enquired, whether light be essentially different from radiant heat? In answer to which I would suggest, that we are not allowed, by the rules of philosophising, to admit two different causes to explain certain effects, if they may be accounted for by one. A beam of radiant heat, emanating from the sun, consists of rays that are differently refrangible. The range of their extent, when dispersed by a prism, begins at violet-coloured light, where they are most refracted, and they have the least efficacy. We have traced these calorific rays throughout the whole extent of the prismatic spectrum; and found their power increasing, while their refrangibility was lessened, as far as to the confines of red-coloured light. But their diminishing -efrangibility, and increasing power, did not stop here; for

we have pursued them a considerable way beyond the prismatic spectrum, into an invisible state, still exerting their increasing energy, with a decrease of refrangibility up to the maximum of their power; and have also traced them to that state where, though still less refracted, their energy, on account, we may suppose, of their now failing density, decreased pretty fast; after which, the invisible thermometrical spectrum, if I may so call it, soon vanished.

The word heat, in its common acceptation, denotes a certain sensation well known to every person. I cannot be misunderstood as meaning that these rays themselves are heat; nor do I in any respect engage myself to show in what manner they produce heat.

From what has been said it follows, that any objections that may be alleged, from the supposed agency of heat in other circumstances than in its state of radiance, or heat-making rays, cannot be admitted against my experiments. For, notwithstanding I may be inclined to believe that all phenomena in which heat is concerned, such as the expansion of bodies, fluidity, congelation, fermentation, friction, &c. as well as heat in its various states of being latent, specific, absolute, or sensible, may be explained on the principle of heat-making rays, and vibrations occasioned by them in the parts of bodies; yet this is not intended, at present, to be any part of what I shall endeavour to establish. I must also remark, that in using the word rays, I do not mean to oppose, much less to countenance, the opinion of those philosophers who still believe that light itself comes to us from the sun, not by rays but by the supposed vibrations of an elastic ether, every where diffused throughout space; I only claim the same privilege for the rays that occasion heat, which they are willing to allow to those that illuminate objects. For, in what manner soever this radiance may be effected, it will be fully proved hereafter that the evidence, either for rays or for vibrations which occasion heat, stands on the same foundation on which the radiance of the illuminating principle, light, is built.

The similar propositions relating to heat, which are intended to be proved in this paper, will stand as follow: 1. Heat, both solar and terrestrial, is a sensation occasioned by rays emanating from candent substances, which have a power of heating bodies. 2. These rays are subject to the laws of reflection. 3. They are also subject to the laws of refraction. 4. They are of different refrangibility. 5. They are liable to be stopped, in certain proportions, when transmitted through

diaphanous bodies. 6. They are liable to be scattered on rough surfaces. 7. They may be supposed, when in a certain state of energy, to have a power of illuminating objects; but this remains to be examined.

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In the view of the apparatus, 1, 2, 3, are the thermometers ; the dotted lines are from the prism at the window; E the coloured spectrum thrown on the table, so as to bring the red colour near the bulbs of the thermometers.

Chemical Experiments on Zoophytes; with some Observations on the component Parts of Membrane. By CHARLES HATCHETT, Esq. F. R. S.-[1800.]

By the experiments, subsequently related, on various shells, crustaceous substances, and bones, it is proved, first, That the porcellaneous shells resemble the enamel of teeth in the.. mode of formation, but that the hardening substance is carbonate of lime.

2dly, That shells composed of nacre or mother of pearl, or approaching to the nature of that substance, and also pearls, resemble bone in a considerable degree, as they consist of a gelatinous, cartilaginous, or membranaceous substance, forming a series of gradations, from a tender and scarcely perceptible jelly to membranes completely organised, in and upon which carbonate of lime is secreted and deposited, after the manner that phosphate of lime is in the bones; and, therefore, as the porcellaneous shells resemble the enamel of teeth, so the shells formed of mother of pearl, &c. in like manner re

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