HALO of the MOON, 175 



moon, of the fmall halo, and of the north-eaflern limb of the 

 large halo, whofe fouth-weftern limb paiTed through the moon, 

 the whole was projected on the horizontal plane, as in the figure 

 already referred to. The moon, a little more than half, is pla- 

 ced in the centre of the fmaller halo ; and both lialos are repre- 

 fented in their true (ituations, relatively to the horizon, and in 

 the circular ftiape which they appeared to have, though they 

 ought perhaps to have been fomewhatforefhortened, and throwa 

 into an elliptic form. 



This halo, as you will fee by the above defcription, appears 

 to be of the kind called by the learned a Corona j and as it 

 fomewhat refembles the famous one of the fun, obferved at 

 Rome in the year 1629, and defcribed by Scheiner*, it deferves 

 the more attention, efpecially as the great halo, on the prefent 

 occafion, having its fouth-weftern limb elevated to the height 

 of 54°, and its north-eaftern depreffed to within 14° of the ho- 

 rizon, was in an oblique position, not eafily reconciled with the 

 theory of Huygens, which feems to require that fuch circles 

 fhould be equally elevated above the horizon all round. It alfo 

 fliews, that Scheiner's original plan of the halo at Rome, 

 which reprefented it as oblique, may have been right, and that 

 Huygens's corredlion, which makes it parallel to the horizon,^ 

 was probably an erroneous conjecflure. 



I am, 



Dear Sir James,^ 



Your humble fervant, 



Will,. Hall» 



* Smith's Optics, voL I. § 534. 



¥IU. 



