498 



NATURE 



[Septembf.r 2o, 1894 



issued by the London School Board some years ago. The 

 author adopted this scheme for use in his school as soon 

 as it was issued, and the experience gained since then 

 has enabled him to produce a thoroughly practical work. ' 

 We know no better work for teaching elementary science 

 to young children. Though designed for Standards I. to ' 

 Vi. of Board and National schools, most of our private, 

 and many of our public, schools would gain by introduc- 

 ing these object lessons into their curricula. 



Precis d: .^ft'h'orologte Endnglne. By F. Canu. (Paris : 

 Gauthier-Villars et Fils, 1S94..) 



L.\ Meteorologie Endogene is, according to the 

 author's definition, concerned with (i) all acoustic and 

 dynamic phenomena produced more or less directly by 

 variations of atmospheric pressure within the earth's 

 crust ; (^) internal manifestations of electricity and 

 magnetism. Ritter gave this branch of knowledge its 

 name, but it was De Rossi who reduced it to a system. 

 M. Canu's volume is an elementary description of ])heno- 

 mena belonging to the physics of the cirth. Among the 

 subjects dealt with are the aurora and its connection 

 with the sun ; earth currents ; subterranean noises, and 

 circumstances affecting them ; terrestrial magnetism ; 

 earthquakes and earth tremors ; and causes producing 

 the escape of fire-damp. All these phenomena are first 

 treated d'>scriptively, and then in relation to other pheno- 

 mena. Thus, after descriptions of the height, spectro- 

 scopic features, acoustic properties, electric character, 

 and geographical distribution of aurora;, we find brief 

 statements of all the causes believed to inrtuence the 

 phenomena. This plan is followed in each chapter, and 

 though the correlation between the phenomena described 

 is sometimes very doubtful, in general the observations 

 quoted deserve consideration. " Pour propager une 

 science," says the author, " il faut avant tout la vul- 

 gariser." To accomplish this object the book has been 

 made casilv understandable to a French-reading public. 



Sack- und Oris- Verzcichnis :u den mineralogischeii tind 

 geoloi;isclicn Arbciteii -'Oii Gerhard voiii Ratlt. Im 

 Auftrage der Frau vom Rath bearbeitet von W. 

 Bruhns und K. Busz. Pp. 197. (Leipzig : Engelmann, 

 1893-) 

 This book is a tribute by the widow of Prof, vom Rath, 

 of Bonn, to the memory of her late husband. It had 

 been her wish to republish his numerous memoirs in a 

 collected edition, but the expense of reproduction of the 

 elaborate crystal drawings with which his researches 

 have been illustrated was found to be prohibitory ; hence 

 the tribute has taken the form of a detailed Index to his 

 works. The plan adopted for the Index is identical 

 with that of the useful Repertorium of the " Zeitschrift 

 fur Krystallographie und .Mineralogie von P. Groth." 

 There are two alphabetically arranged lists, the one a 

 subject-index, the other a locality-index. The crystallo- 

 graphical and mineralogical part is the work of Dr. 

 Jiubz, while for the petrographical and geological part 

 Dr. Bruhns is responsible. The Index gives striking 

 evidence of the vast range of I'rof. vom Rath's studies 

 and observations, while the high standard of excellence 

 which characterised his work is known to all who have 

 occasion to refer to his memoirs. By reason of the 

 diversity of the species and subjects discussed by him, 

 this Index will be of "n-it .ulvant.ni'e to students of 

 mineralogy. 



Eltmenti d: hni.-.t, vols. I. and 1) liy Antonio 

 Roiti. (Florence : Successori Le Monnicr, 1891 and 

 1894-, 

 The first voluine of the third edition of this work was 

 published in 1S91, but the second vohtme. revised and 

 enlarged, h.is only recently appeared. Tlie two con- 

 stitute an admirably-arranged work on general physics, 



NO. I 299, VOL. 50I 



similar in structure to Ganot's " Natural Philosophy. 

 .After an introduction on the properties of matter. Pro 

 Koiti passes to the mechanics of solids, and then to th 

 mechanics of fluids. The ne.xt section is devoted t 

 acoustics, after which come chapters on heat and energ\ 

 These conclude the first volume: the second being cor 

 ceined with radiant energy, and electricity and mag 

 netism. There are nearly nine hundred illustrations i 

 the complete work, but the majority of them are ol 

 friends. However, scientific judgment has been used i 

 making the compilation, and the only matter for con' 

 plaint is the absence cf an index — a common defect t 

 continental publications. In a work of science havin 

 the scope of that under review, such an omission i 

 unpardonable. 



LETTERS TO THE EDITOR. 



[7"/ie Editor doii not kolJ himsel/ resfionsibU for opinions t> 

 pressed by his correspondents. Neither can he undertak 

 to return, or to correspond xcith the writers of^ rejectt 

 manuscripts intended for this or any other part o/ Nature 

 No notice is taken of anonymous communications. ] 



Latitude by«Ex-Meridian. 



The problem under CDnsider.ition is tli.it of finding latitudi 

 by an altilude of a heavenly body taken near the meridian 

 commonly called the " Ex-Meridian." The method most fre 

 quently employed by navigators is that in which a reduction i 

 applied to the observed altitude in order to reduce it to tbi 

 meridian, this reduction being either found by calculation 

 taken by inspection from special tables such as the " Ex 

 Meridian Altitude Tables," by Messrs. lirent, Walter, and 

 Williams. In the following it is proposed to show how this re 

 duct ion may be effected by the use of the Azimuth and Traversi 

 Tables. 



If / be the latitude, d the declination, and // the hour an: 

 the formula of reduction is 



.V = c/<-- (i;i 



where 



C = ^Sll^!^ /"-i^^Godfray's " Astronomy 

 2 Sill (/-</; V sin I / 



Now since C may be considered constant 



dx ox dl = 2, Ckdk : 



Again from the fundamental relation 



cos d cos / cos /; = cos : - sin d sin / 



it is easily 'found that 



dl — dh cos / tan A 



where A is the azimuth, and eliminating C between (I) (2. .i 

 (3) the formula is obtained in the simple form 



X — \ h cos / tan .K 



which expresses the reduction in minutes of arc, // being the 

 number of minutes of arc in the hour angle. 



As an example of the use of this formula, take the observa- 

 tion given at the beginning of the Brent Tables. 



Dale, November 18, D, K. latitude 51' north, hour angle 

 oh. 24m. 54s., declination 19' 20' 43" south, and altitude 

 19° 29' iS". Required the true latitude. 



From Burdwood's Azimuth Tables the bearing is found to be 

 about 6° 15', and we have 



v = iS6'75 cos 51° tan 6' 15' = 12' 48" 



giving a latitude 50° 57' 1 1 " north. 



The result found in the book is 50' 57' 10" north. 



To find the reduction bythcTraver.se Table we may proceed 

 as follows :— Willi 187 as distance and 51' as cour.sc, we have 

 li lat. II 7' 7 ; with this as ./ lal. and 6° 15' as course, we have in 

 the departure column 12' '8, which agrees with the result found 

 above. 



A diflicully attending the above melind ii that the Burdwoou 

 Tables do not cive the azimuths of bodies having an altitude 

 greater than 60', and arc only calculatc<l for bodies whose de- 

 clinations do not exceed the maximum declination of the sun. 

 However, this shouhl hardly be sufficient to condemn the 



