May 1, 1900.] 



KNOWLEDGE. 



107 



the 



will 



degrees of its length every portioa of 

 appear iipou two separate plates. 



The satellit.es of Mars, Jupiter, and Saturn all revolve 

 very nearly iu the equatorial planes of their primaries, 

 and iu the same manner Mercury revolves very nearly 

 in the equatorial plane of the Sun, which is inclined 

 about seven degrees to the plane of the ecliptic. It is, 

 therefore, reasonable to suppose that bodies still nearer 

 to the Sun would revolve in the same plane. It so 

 happens that the Eaith pa.sses through this plane about 

 one week after the date of the solar eclipse of nerl Mav, 

 so that there is a strong probability that if an inter- 

 mercurial planet exists, it will appear somewhere upon 

 the narrow line forming the projection of this plane upon 

 the celestial sphere. It will be seen, therefore, that the 

 date of this eclipse is especially favourable for the pro- 

 posed search. 



We have very good evidence, from the visual observa- 

 tions hitherto made, that no intcrmercurial planet 

 brighter than the third or fourth magnitude exists. Wo 

 possess no evidence whatever for or against the existence 

 of fainter bodies in this region having sufficient size to bo 

 properlv called planets. We are reasonably certain that 

 the immediate vicinity of the Sun is filled with countless 

 bodies of such size as to be properly described as meteors. 

 If we assume that at its average brightness. Mercury 

 is of the fii-st magnitude, and that the albedo of an 

 intcrmercurial planet is the same as that of Mercury, 

 we shall find that at the distance of Mercury from the 

 Sun, a body of the eighth magnitude would be 120 miles 

 in diameter. If its distance from the Sun was but one 

 half as great, its diameter would be 60 miles, and ;f 

 but one quarter as great, or 9,000,000 miles, it would be 

 30 miles in diameter. Judging by the analogous case 

 of Jupiter, the existence of such a small planet is quite 

 possible. 



Should such a body exist, and should it appear upon 

 the plates, which it is proposed to expose somewhere in 

 the State of .Alabama, we should still be entirely at a 

 loss to compute the orbit, or to determine the distance 

 of the body from the Sun. If, however, other photo- 

 graphs of it should be obtained with a similar apparatus, 

 in Spain or Algeria, we should then be enabled to com- 

 pute an approximate orbit, based upon the assuinption 

 that it moved in a circular path. It might then be 

 found again at the following eclipse, which occurs a 

 year later, and a more accurate elliptical orbit could be 

 computed for it. While it is desirable that the duplicate 

 apparatus should be also furnished with four lenses, this 

 IS not necessary, and in case the planet should be found 

 upon our plates, two lenses, one photographing the 

 region on each side of the Sun, would be all that would 

 be necessary to independently make the discovery, and 

 furnish the elements necessary to compute the circular 

 orbit. It is in the hope of inducing some European 

 observer to supply himself with this apparatus that the 

 present article has been written. 



The foregoing plan appears to be of sufficient imoort 

 ance to justify aid from the Observatory. Preparat'ons 

 have, therefore, been made to give it a careful trial It 

 is hoped that this early publication may permit similar 

 obsen'ations to be made at a second station sufficiently 

 distant to reduce the danger of failure from clouds, anu 

 if an intcrmercurial planet should be found, to furnish 

 an approximate determination of the form of its orbit. 



EdW-iVRD C. PICKEEI^G. 



Harvard College Observatory, 

 February 13th, 1900. 



THE PHOTOGRAPHY OF CLOUDS. 



By EUGENli AnTvJNIADI, f.u.a..s. 



(Continued from pnge 81.) 



Some additional references to exposure may bo given in 

 the data accompanying the aniiexed pho(()gra]ihs : — 



*?^r 



« 



Fig. 1. — Cumuli t'oivoastinw fine weatlipr, l.S9i>, .August, 2iW. 9h. 

 45m. a.m., loo;il timiv 



Sulijoiued Plate, Fig. 1. — Cirro-cumulus, following and 

 forecasting wet weather. Photograph taken with the 



FlO. 3. — Pliotograph of the primary and secondary Rainbow?, taken 

 at Juvisy on 1SG9, September, 28d. 51i. 3m., local time. 



