R A I 



R A I 



the rain at a little more than one hundred feet above the level 

 of the fea ; accounts of which have been regularly kept 

 fince the year 1781 ; but previoufly to that period the rain 

 was mealured by an old rain-gage. It is to be regretted, 

 however, that theexaft nature of this invention is not more 

 fully explained ; and that other cheap plain inventions of 

 this nature are not better known to the farmer. 

 RAiN-Z^a/cr. See Water. 



Rains, in the Sea Language, denote all that traft of 

 fea to the northward of the equator, between four and 

 ten degrees of latitude ; and lying between the meridian of 

 Cape Verde and that of the eaftcrnmoil iflands of the fame 

 name. 



It takes its name from the almoft: continual calms, con- 

 ftant rains, and thunder and lightning, to a great degree, al- 

 ways found there. The winds, when they do blow, are 

 only finall uncertain gufts, and (hift about all round the 

 compafs ; fo that (hips are fometimes detained here a long 

 while, and can make hut very little way. 



RAINANGBONG, in Geography, a town of the Bir- 

 man empire, fituated near a river in which there are feveral 

 wells of petroleum, whence its name lignifying a town 

 through which flows a river of earth-oil. N. lat. 20° 26'. 

 E. long. 94'^ 46'. 



RAINBOW, Iris, or, fimply, the bonv, a meteor in 

 form of a parti-coloured arch, or femicircle, exhibited in 

 a rainy Iky, oppofite to the fun, by the refradlion of his 

 rays in the drops of falling rain. 



There is alfo a fecondary, or fainter bow, ulually feen 

 invefting the former at fome diftance. Among naturalifts, 

 we alfo read of /unar rainbows, marine rainbows, &c. 



The rainbow, fir Ifaac Newton obferves, never appears 

 but where it rains in the lun (hine ; and it may be reprefented 

 artificially by contriving water to fall in little drops, like 

 rain, through which the fun (hining, exhibits a bow to a fpec- 

 tator placed between the fun and the drops ; efpecially if a 

 dark body, e. gr. a black cloth, be difpofed beyond the 

 drops. 



That the rainbow is oppofite to the fun, has always been 

 obferved. It was, therefore, natural to imagine, that the 

 colours of it were produced by fome kind of refleftion of 

 the rays of light from drops of rain or vapour. The regu- 

 lar ordi-r of tl>e clouds was another circumilance that could 

 not have efcaped the notice of any perion. But though 

 mere refleftion had in no other cafe been obferved to produce 

 colours, and it could not but have been obferved that refrac- 

 tion is frequently attended with that phenomenon, fo that 

 fome of the ancients, as we Icain frcm Ariilotle's traft on 

 meteors, knew that the rainbow was caufed by the refraftion 

 of the fun's light in drops of falling rain ; yet no perfon 

 feems to have thought of having recourfe to a proper re- 

 fra^ion in this cafe before one Fletcher of Breflaw, who, in 

 a treatife which he publifhed in 157 1, endeavoured to account 

 for the colours of the rainbow by means of a double refrac- 

 tion, and one refledtiou. But he imagined that a ray of 

 light, after entering p drop of rain, and fuffering a refraftion, 

 both at its entrance and exit, was afterwards reflefted from 

 another drop, before it reached the eye of the fpeftator. He 

 feems to have overlooked the refleftion at the farther fide of 

 the drop,orto have imagined that all the bendingsof the light 

 within the drop would not make a fufiicient curvature, to 

 bring the ray of the fun to the eye of the fpe£tator. An- 

 tonio de Dominis, hifhop of Spalato, whofe treatife, " De 

 Radiis Vifus et Lucie," was publiflied by I. Bartolus, in 

 l6il, was the firll perfon who advanced, that the double 

 refraftion of Fletcher, with an intervening reflcftion, was 

 fufiicient to produce the colours of the rainbow, and alfo to 



bring tlie rays that formed t'nem to the eye of the ipefta- 

 tor, without any fubfequent refle6tion. He diiliiiftly de- 

 fcribes the progrefs of a ray of light entering the upper 

 part of the drop, wliere it fuffers one refraction, and after 

 being thereby thrown upon the back part of the inner fur- 

 face, is from thence refli ited to the lower part of the drop ; 

 at which place undergoing a fecond refradf ion, it is thereby 

 bent fo as to come dinftly to the eye. To verify this hy- 

 potliefui, he procured a fm.ill globe of folid glafs, and view- 

 ing it when it was expofcd to the rays of the fun, in the 

 fame manner in which he h%d fuppofed that the drops of 

 rain w ere fituated with refpeft to them, he actually obferved 

 the fame colours which he had feen in the true rainbow, and 

 in the fame order. The tiitory of A. de Dominis was 

 adopted, and in fome degree improved, by Defcartes. 

 Philofophers were, however, for a long time at a lofs when 

 they endeavoured to afiigii realons for all tlie particular co- 

 lours, and for the order of them. Indeed, iiotiiing but the 

 doftrine of the different refrangibihty of the rays of light, 

 which was a difcovery rcferved for tlie great fir Ifaac New- 

 ton, could furnifli a complete folution of this difficulty. 



Dr. Barrow, in his " Leftioncs Optica," (Left. 12. 

 n. 14.) fays, that a friend of his, meaning Mr. Newton, com- 

 municated to liim a method of determining the angle of the 

 rainbow, which was hinted to Newton by Slufuis, witliont 

 making a table of tlie refraftions, as Defcartes did. The 

 dodlor fliews the method, with other curious particulars. 

 But the fubjeft was given more perfeftly by Newton after- 

 wards, in his " Optics," prop. 9 ; where he makes the 

 breadth of the interior bow to be nearly 2° 15', that of the 

 exterior 3"^ 40', their diilance 8° 25', the grcatell femidiame- 

 terof the interior bow 42^ 17', and the lead of the exterior 

 50^ 42', when their colours appear ftrcng and perfcft. 



Rainbow, Theory of the. To conceive the origin of the 

 rainbow, let us confider what will befall rays of light 

 coming from a very remote body, e. gr. the fun, and 

 falling on a globe of water, fuch as we know a drop of rain 

 to be. 



Suppofe, tlien, A D K N ( Plate XVIII. Optici, Jig. 1.) 

 to be a drop of rain, and tl;e lines E F, B A, ON, to be 

 rays of light coming from the centre of the fun ; which, on 

 account of the immenfe diltance of the fun, we conceive to 

 be parallel. 



Now the ray B A being the only one that falls perpen- 

 dicularly on the furface of the water, and all the reft ob- 

 liquely, it is eafily inferred, that all the other rays will be 

 refracted towards the perpendicular. (See Refraction.) 

 Thus the ray E F, and others accompanying it, will not 

 go on ftraight to G ; but as they arrive at H I, they 

 will defleft from F to K ; where fome of them, probably, 

 efcaping into the air, the reft are reflefted upon the line 

 K N, fo as to make the angles of incidence and refleftion 

 equal. 



Farther, as the ray K N, and thofe accompanying it, 

 fall obliquely upon the furface of the globule, they cannot 

 pafs out into the air, without being refrafted, fo as to recede 

 from the perpendicular L M ; and, therefore, they will not 

 proceed ilraight to Y, but will defieft to P. 



It may be here obferved, that fome of the ray?, arriving 

 at N, do not pafs out into the air, but are again reflefted to 

 Q ; where being refrafted, like the reft, they do not pro- 

 ceed right to Z ; but, declining from the perpendicular 

 T V, are carried to R ; but iince we here only regard the 

 rays as they may affeft the eye placed a little below the 

 drop, e. gr. at P, thofe which defleft from N to Q, we 

 fet afide, as ufelefs ; becaufe they never come to the eye. 

 On the contrary, it is to be obferved, that there are other 



rays, 



