Page:Cyclopaedia, Chambers - Volume 2.djvu/571

RAI Idea whereof will not be unacceptable.

Upon measuring, then, the Rain falling yearly ; its Depth, at a Medium, is found as in the following Table.

Depth of Rain falling yearly, and its Proportion in several Places.

Pportions of the of several years to one another.

Proportion of the of the several Seasons to one another.

Praænatural-, as of blood, &c. are very frequent in our Annals; and even Natural Histories, yet; is strictly pried into, will be all found other things than Rain.

Bloody Rains, Dr. Merret observes, are, certainly, nothing else but the Excrements of Insects.

Accordingly, Gassendus gives an Instance of a bloody Rain in France, which terrified the People; but which Peirisc found to be only red Drops coming from a fort of Butterfly that flew about in great Numbers, as he concluded from seeing such red Drops come from them; from the Drops not being laid on Buildings, or the outer Surfaces of Stones, &c. but in Cavities and Holes; and from those Walls only being tinged therewith that were next the Fields, not those in the Streets; and the first only to a little Height, such as Butterflies are used to fly to.

The same Dr. Merret adds, that 'tis most evident the Rains of Wheat are nothing but Ivy-Berries, swallowed by the Starlings, and again cast forth by Stool.

An Instance of such a Rain we have in the ''Philsoph. Trans.'' from the Country about Bristol, by Mr. W. Cole; who, upon examining the Drops, found them to be the Seeds of Ivy-Berries, blown down by fierce Winds horn Towers, Churches, Chimneys, Walls, &c. where they had been left by Birds, chiefly Starlings and Choughs.

The French have a Tradition of a Rain of Stones, in a Plain six or seven Leagues long between Arles and Marseilles, call'd la Crau, which is now quite cover'd therewith.

The Fable has it, that Hercules in his Engagement with Albion and Bregma, in Favour of Neptune; wanting Darts, was assisted by Jupiter with a Shower of these Stones, seen to this Day. Another Account of their Origin, see under the Article.

, in the Sea Language, is all that Tract of Sea to the Northward of the Equator, between 4 and 10 Degrees of Latitude; and lying between the Meridian of Cape Verde, and that of the Eastermost Islands of the same Name.

It takes its Name from the almost continual Calms, constant Rains, and Thunder and Lightning to a great Degree, found there. The Winds, when they do blow, are only small uncertain Gusts, and shift about all round the Compass; so that Ships are sometimes here detain'd a long while, and can make but little way.

RAIN-BOW,, or simply, the Bow, a Meteor in form of a party-coloured Arch or Semicircle, exhibited in a rainy Skie, opposite to the Sun; by the refraction of his Rays in the Drops of falling Rain. See Meteor, Rain, and Refraction.

There is also a secondary or fainter Bow, usually seen investing the former, at some Distance; and among Naturalists we read of Lunar Rainbows, Marine Rainbows, &c.

The Rainbow, Sir Isaac Newton observes, never appears but where it rains in the Sun-shine; and may be represented artificially, by contriving Water to fall in little Drops like Rain thro' which the Sun shining, exhibits a Bow to a Spectator placed between the Sun and the Drops; especially if a dark Body, e. gr . a black Cloath be disposed beyond the Drops.

Anton, de Dominis first accounted for the Rainbow, in 1611 He explain'd at large how it was form'd, by refraction and reflection of the Sun-beams in spherical Drops of Water; and con- firm's his Explications by Experiments made with Glass globes, &c. full of Water. Wherein he was follow'd by Des Cartes, who mended and improved on his Account: But as they were both in the Dark as to the true Origin of Colours, their Explications are Defective, and in some things erroneous; which 'tis one of the Glories of the Newtonian Doctrine of Colours, to supply and correct.

To conceive the Origin of the Rainbow, let us consider what will befal Rays of Light coming from a very remote Body, e. gr. the Sun; and falling on a Globe of Water, such as we know a Drop of Rain to be.

Suppose then ADKN (Tab. Opticks, Fig. 45.) to be a Drop of Rain, and the Lines EF, BA, ON, to be Rays of Light coming from the Centre of the Sun; which, by reason of the immense Distance of the Sun, we conceive to be Parallel. See Parallel

Now the Ray B A being the only one that falls perpendicularly on the Surface of the Water; and all the rest obliquely; 'tis easily inferr'd that all the other Rays will be refracted towards the Perpendicular. See Refraction.

Thus the Ray EF, and others accompanying it, won't go on strait to G ; but as they arrive at HI, deflect from F to K, where some of them, probably, escaping into the Air, the rest are reflected upon the Line KN, so as to make the Angles of Incidence and Reflexion equal. Sec Reflexion.

Further, as the Ray KN, and those accompanying it, fall obliquely upon the Surface of the Globule; they cannot pass out into the Air, without being refracted, so as to recede from the Perpendicular LM; and therefore will not proceed straight to Y, but deflect to P.

It may be here observ'd, that some of the Rays arriving at P, do not pass out into the Air, bur are again reflected to Q; where being refracted like the rest, they do not proceed right to Z, but declining from the Perpendicular TV, are carried to R: But since we here only regard the Rays as they may affect the Eye placed a little below the Drop, e. gr. at P, those which deflect from N to Q, we set aside as useless, because they never come at the Eye. On the contrary, it is to be observ'd, that there are other Rays, as 2, 3, and the like, which being reflected from 3 to 4, thence to 5, and from 5 to 6 may at length arrive at the Eye placed beneath the Drop.

Thus much is obvious : But to determine precisely the Quantities of Refraction of each Ray, there must be a Calculation : By such Calculation it appears that the Rays which fall on the Quadrant AD, are continued in Lines, like those here drawn in the Drop AD KN; wherein there are three things very considerable: First, That the two Refractions of the Rays in their Ingress and Egress are both the same Way, so that the latter does not destroy the effect of the former. Secondly, That of all the Rays patting out of AN; NP, and those adjoining to it, are the only ones capable of affecting the Sense; as being sufficiently close or contiguous; and because coming out parallel; whereas the rest are divaricated, and dispers'd too far to have any sensible Effect, at least to produce any thing so vivid as the Colours of the Bow. Thirdly, That the Ray NP has Shade or Darkness under it: For since thereis no Ray comes cut of the Surface NA, 'tis the same thing as it the Part were cover'd with an Opake Body. We might add, that the same Ray NP has Darkness above it; since the Rays that are above it are ineffectual; and signify no more than if there were none at all.

Add to these, that all the effectual Rays have the same Point of Reflection, i. e. the parallel and contiguous Rays; which alone are effectual after Refraction, will all meet in the same Point of the Circumference; and be reflected thence to the Eye.

Further it appears by Calculation, that the Angle ONP, included between the Ray NP, and tile Line ON drawn from the Centre of the Sun, which is the Angle whereby the Rainbow is distant from the opposite Point of the Sun, and which makes the Semidiameter of the Bow; contains 41° 30′. The Method of determining it see hereafter.

But since betide those Rays coming from the Centre of the Sun to the Drop of Water, there are many more from the several Points of its Surface; there are a great many other effectual Rays to be considered; especially that from the uppermost, and that from the lowest Parr of the Sun's Body.

Since then, the apparent Diameter of the Sun is abound Se- conds, it follows that an effectual Ray from the upper Part of the Sun will fall higher than the Ray EF, by 16 Seconds: This does the Ray GH, (Fig. 46.) which being refracted as much as E ; deflects to I, thence to L; and at length emerging equally refracted with the Ray NP, proceeds to M; and makes an Angle ONM, of 41° 14′ with the Line ON.

In like Manner the effectual Ray QR coming from the lowest Part of the Sun, falls on the Point R, 16 Minutes lower than the Point F, on which the Ray EF falls; and being refracted declines to S; whence it is reflected to T; where emerging into the Air, it proceeds to V; so, as the Line TV, and the Ray OT contain an Angle of 41° and 46′. Again,