September 20, 1S94] 



NA TURE 



499 



nethod, especially as the sun is the body most frequently 

 observed. 



It may be also of interest to notice an addiiional u«e of the 

 Brent Tables. These are constructed on formula (i). Table III. 

 (■iving the value of C for every decree of latitude from o' to 

 ;o', and of declination from o' to 60'. Now (2) may be written 

 in the form 



J "''' 



So that if we wish to find during what lime observal ions m.iy be 

 taken so that an error dh in the estimated longituile will not 

 produce more than an error dl in the latitude, we have, if .' be 

 expressed in time, 



C ./// 



Thus in the abave example, suppose it were required to find 

 during what time observations should be laken, so that an error 

 of a second of time in the estimated longitude would not produce 

 more than an error of a second of arc in the latitude, we 

 have 



2/i = 





t = 



= 12m. IIS. 



I 2j 



In cases where the latitude and declination are of the same name 

 and do not differ by very much, this time is very small, out when 

 uf different names in high latitudes / is considerable. 



I. White. 

 H.M.S. I/au'ie, Mediterranean Squadron. 



Magnetism of Rock Pinnacles. 

 Owi.N'G to my absence from home, I have only just seen the 

 letters of the Rev. K. IIiII, M.M..S., and James Heelis, in 

 NA'ri;i;E of August 2 and 9, on tlie above subject. The writers 

 have apparently overlooked the very interesting report by 

 Profs. Rucker and Thorpe, published in the lirit. .Assoc. 

 Report (or 1SS9, p. 5S6, in which it is sho«n that "all the 

 principal masses of basalt in ihe kiiiijdom form centres of 

 magnetic a'tractlori," and that "the .Malvern Hills, though 

 composed of diorite in which magnetic polarity can barely be 

 detected, produce deviations of t^venty minutes of arc at a 

 distance of one mile from their axis.' 



The mineral magneiite is an original constituent of basic 

 igneous rocks ; and, ow ing 10 the action of gravity on this heavy 

 mineral whilst the magma which contained it was sliil in a fluid 

 or plastic condition, or lo some other cause, it has sometimes 

 >egregated into masses, or has become more or less concentrated, 

 in certain parts of igneous rocks. Two very interesting papers 

 on gabbro-, in which remarkable concentrations of magnetite 

 have been observed, have quite recently been read before the 

 Geological Society — one by Sir Archibald Geikieand .Mr. Teal), 

 and iheotherby Mr. Alfred llarlcer — in which the concenlration 

 observed in these rocks is accounted for in ditterent ways. Rocks 

 in which a local concenlration of magneiite has tai;en place must 

 have a veiy powerful eflect on the magnet even at a distance. 



In addition to original magnetite, basic rocks, especially those 

 of igneous origin, contain secondary in.agnetiie, and magnetic 

 pyrites, formed by aqueous and other agents, out of the nnsiable 

 minerals of which the original rocks were built up. Serpentine, 

 tor instance, usually contains secondary magnetite formed out of 

 the mineral olivine, one of the principal cunstiluenis of the 

 peiidoliie from which serpentine was derived. 



Owing to the presence of the above original and secondary 

 minerals, small h tnd-spicimens of ordinary igneous locks — even 

 those in which special segregation oi m.ignetite has not taken 

 place — will generally be found, when examined, 10 attract a 

 magnet more or less powerfully. 



A suitable instrument for testing hand-specimens may be 

 formed by allaching a small hoise-shoe magnet to one arm of a 

 chemical balance. After the equilibrium of the balance has 

 been restored, place the hand-specimen under the magnet and 

 raise it carefully. The balance will dip unmistakably towards 

 the specimen it it contains an appreciable amount of magneiite. 



20 Nevern Square, S.W. C. A. .McMahon. 



Aurora. 



In Hairhead, Renfrewshire, on l''rid.\y (14th), at 9.15 p.m., I 

 witnessed ihe finest aurora I have observed (oryeirs. I'he luinm- 

 ous arch extended from sjuih-west to north-iasr, and shortly 



NO. 1299, VOL. 50] 



reached the zenith. The rapid fluctu.ilions In the streamers were 

 remarkable. There were no coloured hands. The moon, nearly 

 full, was shining, rendering the appearance less vivid. In about 

 fifteen minutes ihe auroral light began 10 wane. 



Tynron, Dumfriesshire, September iS. J. Sh.WV. 



BRIGHT PIiOJECT/0.\S ON MARS' 

 TERM IN A TOR. 

 '"P H F. appearance of bright spots on the surface of 

 -•- iMars has been long familiar to observers of this 

 planet. An idea of the ease with which they may be 

 observed can be gathered from the following words of 

 SchiaparelIi,our highest authority on Martian questions. 



'• It would not be difficult to find a series of hypotheses 

 which would explain satisfactorily the appearance of the 

 polar and other white spots by attributing them in some 

 way to the evaporation of the supposed seas, and to the 

 atmosphere of the planet whose existence is indisputable. 

 But I consider it more useful to point out that these 

 difierent white spots are, of all the species of appear- 

 ances on .Uiirs, the easiest to observe. They require 

 only an instrument of moderate power and a very per- 

 severing attention. The .... peculiarities concerning 

 these spots show that they offer a field for the most interest- 

 ing investigations, whose importance in the study ot the 

 physical constitution of Mars is obvious ; and in this 

 field useful work could be done by those observers who 

 are not able to decipher the inucli more dillicult details 

 of the canals and their doubling." 



Now the appearances of some of these spots in 

 different positions on the planet's disc have been 

 observed at times to undergo rapid changes in bright- 

 ness, and it was, if we do not err, the distinguished 

 observer just quoted who first pointed out the tendency 

 of some of these bright regions to increase relatively 

 in brightness as the terminator of the planet was 

 approached. 



Observations of more recent date than those just 

 referred to, have, however, made us acquainted with 

 other surface phenomena connected, perhaps, in some 

 way with, but of more importance than, the bright spots, 

 and these are the bright prominences or projections at 

 the terminator. 



It must be remembered, nevertheless, that bright pro- 

 jections may be of two kinds, optical and real. 



The former is an effect of contrast. It may be brought 

 about by the approach of a very bright spot to the 

 terminator where the adjacent darkness tends to give it 

 the appearance of a projection, or, in other words, it is 

 the result of pure irradiation. .-Vs a somewhat parallel 

 example may be mentioned the "drop'' seen at the 

 transits of Venus. That numbers of such spots have 

 been seen at various times, can easily be shown by a 

 brief examination of the records. Terby, for instance, 

 in 1 888, on several nights watched three such points, 

 which, as they approached the western edge oi the disc, 

 became very bright, and before passing behind the 

 planet, projected beyond the edge of the disc, as was the 

 case with the polar cap. .At .Mount Hamilton, also, 

 numerous similar observations at various times have 

 been made. 



The second kind of bright projection is that due to 

 the physical peculiarities at the surface of the .Martian 

 globe Itself, and may correspond to elevated highly 

 illuminated regions. These were first observed at the 

 Lick Observatory in 1S90, at the Observatory at Nice, 

 and at the Aret[uipa Observatory in 1 89;. The first 

 prominences observed this year were seen on June 28 at 

 Mount Hamilton, and since then have been more or less 

 constantly observed. 



To give the reader an idea of what actually is seen at 

 the telescope when such a projection is under observa- 

 tion, an instance or two may not be out of place. 



