RAINBOW. 



observation the cylinder must be emp- 

 tied. 



A very simple rain-gauge, and one 

 which will answer all practical purposes, 

 consists of a copper funnel, the area of 

 whose opening is exactly ten square inch- 

 es : this funnel is fixed in a bottle, and 

 the quantity of rain caught is ascertained 

 by multiplying the weight in ounces by 

 .173, which gives the depth in inches and 

 parts of an inch. In fixing these gauges, 

 care must be taken that the rain may 

 have free access to them : hence the tops 

 of buildings are usually the best places. 

 When the quantities of rain collected in 

 them at different places are compared, 

 the instruments ought to be fixed at the 

 same heights above the ground at both 

 places, because, at different heights, the 

 quantities are always different, even at 

 the same place. 



RAINBOW. The rainbow is a circular 

 image of the sun, variously coloured. It 

 is thus produced : the solar rays, entering 

 the drops of falling rain, are refracted to 

 their further surfaces, and thence, by one 

 or more reflections, transmitted to the 

 eye : at their emergence from the drop, 

 as well as at their entrance, they suffer a 

 refraction, by which the rays are separat- 

 ed into their different colours, and these, 

 therefore, are exhibited to an eye pro- 

 perly placed to receive them. That this 

 is the true account of the formation of 

 the rainbow, appears from the following 

 considerations: 1, That a bow is never 

 seen but when rain is falling, and the sun 

 shining at the same time, and that the 

 sun and bow are always in opposite quar- 

 ters of the heavens : this every one's ex- 

 perience can testify. 2. That the same 

 appearance can be artificially represent- 

 ed by means of water thrown into the 

 air, when the spectator is placed in a pro- 

 per position with his back turned to the 

 sun : experiment will shew this. 3. That 

 its formation, as above described, can be 

 clearly explained from the properties of 

 light, alreadv demonstrated in the former 

 parts of this dictionary. 



Let A B, (Plate XIII. Misccl. fig. 10) 

 be a drop of water, and CD, a pencil of 

 solar rays incident thereon ; if all the 

 rays of any one colour, as red, belonging 

 to the pencil CD, be refracted to^the 

 same point, G, and thence reflected, they 

 will fall on the space, R Q, with the same 

 obliquity, and at the sumo distances from 

 each oilier as the refracted rays, if pro- 

 ceeding backward from G, would fall on 

 the space, T S, but these, at their refrac- 

 tion, would emerge into TI>, C S, &c. 



parallel to each other; v the rays, GR, 

 G Q, will emerge from the drop parallel 

 to each other, and therefore will enter an 

 eye properly placed copiously enough to 

 cause a sensation ; a red colour will 

 therefore appear in the direction of these 

 rays, and so of others. But if the re- 

 fracted rays do not meet in the same 

 point, the reflected rays, (fig. 11) IV, 

 PQ, will not fall on the surface at the same 

 distance from each other that P T and 

 I S do, though their obliquity to the sur- 

 face be equal to that of the latter; there- 

 fore, the refracted rays will emerge, 

 diverging from each other, and conse- 

 quently will not enter the eye copiously 

 enough to cause a perception of their co- 

 lour. It is plain, that where the rays of 

 any colour emerge parallel, all these 

 emerging rays will be inclined to the in- 

 cident rays in the same angle. And by 

 calculation it is found, that the red rays, 

 when they emerge parallel to each other, 

 make with the incident rays an angle, 

 ABO, (fig. 12) of 42 2', and the violet 

 an angle, A C O, of 40 17', and the rays 

 of the other colours, angles greater than 

 the latter, and less than the former. 



If through the eye which receives the 

 emerging rays, there be drawn a line, 

 A X, parallel to the incident rays, it will 

 make, with the emerging rays of each 

 colour, angles, R A X, and VAX, &c. 

 equal to the above. This line, A X, is 

 called the axis of vision. The several 

 drops placed in the lines, A R, A V, &c. 

 will exhibit to the eye at A, the several 

 prismatic colours respectively, as appears 

 from what has been said ; and if those 

 lines be supposed to revolve with a coni- 

 cal motion round the axis of vision, it is 

 evident, for the same reason, that all the 

 drops placed in each of the conic sur- 

 faces, so generated, will transmit the 

 rays of each colour respectively to the 

 eye, and therefore, that a number of cir- 

 cular, concentric arches of the prismatic 

 colours, adjoining to each other, will be 

 exhibited to the eye. This explanation 

 relates to the interior bow, whose co- 

 lours, beginning from the outside, are 

 red, orange, &c. as in (he prismatic spec- 

 trum. This bow can never be seen if 

 the sun be elevated more than 42 2' 

 above the horizon ; for the horizon, II O, 

 (fig. 13) always makes with the axis of 

 vision, A X, an angle equal to the eleva- 

 tion of the sun, v in the case here staled, 

 the line, A Q, marking the vertex of R 

 rainbow, would fall entirely below the 

 horizon. As the anterior bow is formed 

 by one reflection and two refractions, the 



