272 



SCIENCE. 



[N. S. Vol. IV. No. 87. 



On relief maps vertical exaggeration is ex- 

 cusable and without doubt necessary, but it can 

 hardly be said to be in text-book illustrations. 

 Natural scenery is sufficiently imposing not to 

 need to be made attractive by exaggeration, 

 while correct illustrations strengthen the pupil's 

 confidence in the truth of what he is taught. 



I append a view of Popocatapetl as it is 

 represented in a modern geography in common 

 use in our schools, and, for comparison, a profile 

 drawn from a photograph of the volcano as it 

 appears from the valley of Puebla. 



Oliver C. Farrington. 



Field Columbian Museum, Chicago, III. 



ON THE notation OF TERRESTRIAL MAGNETIC 

 QUANTITIES. 



At the International Meteorological Congress 

 to be held in Paris, a number of questions of 

 special interest to magneticians have been pro- 

 posed for discussion, among which is the follow- 

 ing: The same notation should generally be 

 employed, H for horizontal force, X for the 

 northern component, F for the western com- 

 ponent, Z for the vertical force, and V for the 

 potential. As the need of some uniform nota- 

 tion has been made apparent to me in connec 

 tion with the journal Terrestrial Magnetism, I 

 have been paying this matter some attention 

 with the view of obtaining a concise and logical 

 system for adoption in this journal. 



The principle upon which I proceed is to take 

 the first letter of a word designating a particular 

 quantity, if at the same time it conforms with 

 typographic requirements, such, for example, 

 as declination, which is common to several lan- 

 guages. In this way I have thus far obtained 

 the following : D for declination, J for inclina- 

 tion, S for horizontal component of force, V 

 for vertical component, F for total force. 



Upon examination it will be found that 

 these letters stand for words derived in almost 

 all cases originally from the Greek and Latin 

 languages and, with but insignificant variations 

 in spelling, common to several of the main 

 modern languages. The Germans will be asked 

 to yield a point with regard to F,* but this, as 



*The initial letter of the German word Kraft is 

 frequently used to designate the moment of inertia 

 and hence will not answer for force. 



will be seen below, will be made up to them in 

 the adoption of G for magnetic potential. V, 

 taken from the Latin vis or I from intensitas, or 

 D from the Greek word dvva/uig, would not do 

 for force, as they are already taken. Nor 

 would T from totus or P from Trdf answer, since 

 the former is frequently used for time of vibra- 

 tion, and so in fact is the letter P, which stands 

 besides for the first deflection coefficient. As I 

 hope to be able to find satisfactory notation for 

 all the principal magnetic quantities, I am keep- 

 ing this matter constantly in mind in adopting 

 any particular letter. The English and French 

 have force, and I have, therefore, adopted F for 

 total force. As it is frequently the custom to 

 designate angular quantities by Greek letters, I 

 should have preferred, had it been possible, 

 to adopt S and t instead of D and I, but the 

 Greek i is a very unsatisfactory letter from a 

 typographical standpoint. Moreover, if found 

 desirable later on, the small letters d and i or 

 6 and i can be reserved for the variations on the 

 mean of day and on the mean of year respect- 

 ively. 



I think it very much to be deplored if Z, as 

 above proposed, be universally adopted to des- 

 ignate the vertical force. It should not be for- 

 gotten that the Gaussian mode of resolving the 

 magnetic force into northerly component (X), 

 westerly component (Y) and vertical component 

 (Z) applies to a local system of coordinates, not 

 to a fixed system, as the layman might natur- 

 ally suppose, a fact which is even apparently 

 forgotten at times by magneticians. The mean 

 values of these components for a complete cir- 

 cuit of the earth along a parallel of latitude can, 

 in consequence, no more be physically inter- 

 preted than the mean H, for example. I am 

 therefore opposed to adopting a letter for the 

 vertical force which in no way gives evidence 

 of the exact quantity for which it stands. F, 

 on the other hand, is logically connected with 

 S and at the same time implies that the direc- 

 tion of the quantity that it symbolizes is local, 

 the direction of the vertical or plumb line vary- 

 ing from point to point. 



For the same reasons I am not in favor of 

 adopting X for northerly component and Y for 

 westerly component. Let authors choose this 

 method of notation if they prefer it, but in a 



