150 



Proceedings of Learned Societies. 



[Sept. I, 



«ro dfpencfenee on the pofition of thoi'e 

 luitrinaiies relatively to one another, but 

 fc.ii.>p«ns, it lliould i'eem, conftantly at the 

 i-m« Ivour, in ali afptds cf the Mt,on, 

 ajvl at all (eaft>ns of the year. 



The io liwing is an account of two in- 

 tcrelHng lainbows leen at Dungals j as 

 cmmunkated to the Royal Socie'y of 

 E.lihbargh>, by Mr, ProfelTor PlaYFaIr. 



At Dungials, a little before funfet, w.,s 

 fcea a large and beaiitiful raiiibowj form- 

 *A on a cloud which hung over the lea, 

 and from '.vhtclv a fhower waii falling at a 

 tij-nkiersble diilance to the S. E. The 

 &'.;a was about 2° high, fo that the arch 

 was not mcth lefs than a f niici.'cle witli 

 iis higFiell point elevated about 4.0°. At 

 sHe point \v]iere the northern extremity of 

 tiiis arch touched thehoiizon, anotlier arch 

 fsenned alfo to fpriiig from the fea, diverg- 

 iiic; from the former at an angle of 3° or 

 4'*^ <n the fide towards the Sun. This 

 31'ch tiid not txteed 70 or S° in lcn.gih ; 

 iit was of the fame breadth with the prin- 

 cipal bow ; it had the colours in the fame 

 esder, and nearly of the fame brightnefs ; 

 <i if any difference wasdifcernible, it was, 

 euat ihe tranfition from one colour to an- 

 «th<T was not made with (o much delicacy 

 ia tlie laft-meutioned rainbow as in the 

 foijjier. 



A phenorejenon fimilar to this is defcrib- 

 cif ii> the " Philofophical Tranfaflioiis," 

 mhith is afcribed to the rcflcfiion of the 

 Stm's rays from the furface of the fea, fo 

 as to tall on the cloud where the rambo-w 

 was formed. This hypoihtils feemed to 

 Mr. P. to agree with the phenomenon 

 ■witntffed at Dunglafi : for the accidental 

 riisibow was leen only at the extremity 

 where ths principal arch rofe from the 

 lea, and where the Sun's rays, refleited 

 fs-om the fuilace of the water, might fall 

 em the drops of rain. The other parts of 

 the cloud could not receive rays fo refttft- 

 ed, as the land ir^tervened, and theie, ac- 

 eordiagJy, no veftige of the accidental 

 wlnbowwas obferved. 



The accidental rainbow lay an the fide 

 t'ward the Sun; which is agreeable to 

 ii;r liypothefis ; for the rays that fell, af- 

 ter itflcilion from the furface cf the waier, 

 en the drops of rain, mulf have come as 

 it<m R point as much deprrfled below ihe 

 i.^r.zon, as the Sun was, at that inftant, 

 ekv:ited above it. The ?.xis of the acci- 

 4!tri(.l rainbow muft, therefore, have muJe 

 »i h the axis of the principal one, an 

 2n.;!e equal to twice the Sun's elevation, 

 iiwJ iis centre mult have been elevated by 



that fame qnantity above the centre of the 

 other ; fo that if it had been complete, it 

 would have been wholly between the prin- 

 cipal rainbow and the Sun. 



When phenomena of this kind occur, 

 Mr. P. recommends, that the inclination 

 of tile two bows and the Sun's height 

 fliould be obferved at the fame time. For 

 if / be cdl d their angle of interfeflion, 

 E the elevation of the Sun, and 5 the angle 

 fiibtcnded at the eye by the femidiameter 

 of the rainbow, if complete — an angle 

 which is conflantty the fame, and nearly 

 equal to 41", it is tafy to infer from fphe- 



llne E 



rical trigonometry that fine |- J = 



fine S. 

 C'lmputing from this formula, the incli- 

 nation of the two bows, in the prefent 

 inftance, comes out nearly 5". 



Phenomena of this kind can but feldom 

 occur, as the neceflary conditions will not 

 often come together. The principal rain- 

 bow mult be over the fea, the furface of 

 which nauft be fmooth, and extend fome- 

 what on the fide towards the Sun, and the 

 Sun fo low that the light reflefted from the 

 water may beconfiderable. 



Sir George Mackenzie read to this 

 Society a Paper containing an account of 

 experiments on the combuftion of the dia- 

 mond, of which the following appears to 

 be entirely new, and tends to prove the 

 identity of carbon and diamond. 



Having prepared fome pure oxide of 

 iron, he mixed a fmall quantity of it v»ith 

 one-fourth of its weight of diamond- 

 powJer, prepaied in ihe following man- 

 ner : — The diamond, being reduced to 

 powder in a fteel mortar, was boiled in 

 muriatlc-acid, to diffolve the iron parti- 

 cles which have been abraded from it.— . 

 After proper edulcoration, it was heated 

 in a muffle, to burn oflF the carbon of the 

 fteel which remained after treatment with 

 the acid, and which rendered the powder 

 of a grey colour. He obferved the coaly 

 matter take fire at the e<lge of the heap of 

 powder next the ftrongell heat, and gra. 

 dually fprend it felt, till at laft the whole 

 appeared as if burning. The glow through 

 t;ie powiler ceafed foon after, and on re- 

 moving it he found it perfectly clean and 

 white. From the diminution of the ori- 

 ginal weight of the diamond, he found 

 that a part of it had been confumed. 



The mixture of oxide and diamond- 

 pnwder thus pre^jared was put into a Cor- 

 nifli clay crucible, and expofed to a pretty 

 llrjng heat for half an hour, after which. 



tht 



