54 
On the Different Methods 
[Feb. 
This is sufficiently obvious. The question is, bow we are to regulate our shading so 
as to produce an accurate representation of all the inequalities of the ground we are 
employed in mapping ? To answer this question, we must first settle the direction 
in which the light is supposed to fall, as on that circumstance the arrangement of 
the shadows depends. 
Out of the many directions which might he assumed, two only have been gene- 
rally adopted. In the one, the rays of light are supposed to fall at an angle of 45° 
rom the N. U . corner of the map, the shadows being all directed south-east; and 
those planes facing the N.W. at an angle of 45°, being in brilliant white light. In 
ie . °, ! j- K ? nice tu ^£kt is assumed to be in the same position with the eve, at 
n lnt.e 'iiia ( i> a nee and the rays, therefore, fall perpendicularly on the map. Every 
I surface will, m this method, have a brilliant white light, while perpendicu- 
< . ui faces alone will be quite black. I ought to observe, that I use the word sha- 
„ I 0 m ? re °\ J f ss deprivation of light, and not as indicating what 
arc commonly called projected shadows* 
tlAnf A ^ [ ne ' lJO '° methods is that practised by the draughtsmen attached to 
nj' , J ance Survey, and by them taught to such of the Company’s Engineer 
i J ! re flowed to attend that survey. When well executed, it has'a fine 
thP for^fp TAAi S ° f me R P ecn " ens 1 have seen executed from models, emulated all 
of aer nl n” d AA* 1 0t - a tt"® 1 representation. The gradation of tints or effect 
chms perspective, is added to make the deception the greater, hut I think injudi- 
and ®M 1Ch A ' takeS from t,ie geometrical simplicity of the principle, 
* ^ er, ders it less subject to the regulation of the scale and compass. This latter 
‘ ’ 1 m> opinion, absolutely necessary to attaining that precision both in the re- 
alone iltl ° a an< P erusa ^» which can never he the result of judgment by the eye 
Another objection to it, in my opinion, is that, properly speaking, all the plain 
country should h e shaded, and the white paper only left for those surfaces which, 
icing inclined at an angle of 45°, face the north-west quarter. For as the number 
wwX S tt o r i u" • <1Ual P° rtioas of surface will he as the sines of inclination at 
f, , fa r’ V; [ he mcUnation of surface + 45° ; so in horizontal sur- 
surface V 45 the l ,ro n rti »“ wil1 be ?» sin. 45«: 1. But the inclined 
The lielit uni tl, i 11 os&er one in the proportion of the co-sine of inclination. 
tto ’Krte wll/h "e’rtreTeXni'c 0 ? so 
cos. 45° • ] Now in nr, ^ *. , n t,ic tl1,ove » and will be equal to sin. 45 
£as this p„V„U,ee„ mZ X % [Tf — “T'V »“ ">« Principle, 
are aware of such a result those ^Oow it, 
school, ^le^nds^orTuie su^ipo^itimfThat ravs ^of Tiel t^f ,? raCtice ° f tl,e Gerl,,M 
surface. Projected shadTs . XrefarJ Mere w ll ^endicularly on the 
will have a gradation of shade proportional to tl, > l • .? e > but every surface 
by its inclination to the direction if the rays. It it KkTin elfart 
as a picture, inasmuch as where the steepness is enii .l ii , eft . e , a ’ c ° nS “l ered 
the shading- is the same. But it appears better adapted fm- V"° ' 1< eS ot a ndge ’ 
winch is to afford precise information. Now in a man e\pr A P ur P° s f s ot . a ina P» 
the riernsipi' nf It L-nmrr fi.„, u ■. _ ia P executed on this principle, 
the peruser of it knows that every equally deep shade is *^ cute “ ° U this l )rinci i ,k 
point of the utmost consequence in a mihtaA dnw T V 1 "*"* ^ ^urfiice,- 
gathered by guess or loose inference from a m h - ’ one which can only b 
precise, arc the Germans in their practice of thc\Ltho™?h 1 ^° Enfr,is . U st >’ K ’* * 
to white, or ink to paper in these shades, where the ! ‘ the proportion of bla 
regulated by the consideration of the light which wnn?ni° hatching 13 used, 
pared with a horizontal o,,p th,. 1,01,,.. o • ( trill on each surface, coi 
,' ua P executed in the English style. So 
of bl abk 
pared with a horizontal one, the hatter beinn renresnAtAi A' 1 ? 11 e i‘. ch surface com- 
paper. Thus, from white to black we may have an inr 1 ' 3 *} f U Jl &ht or the white 
accurately represent! ngcor responding inclina 01SZ p " 6 !? 16 series of Guides, all 
and perpendicular. If we suppose the lines * u ?,° antl lJ0 °> horizontal 
number may he as the diminution of light i. P 1 0t l ! ie ® am ® force, then their 
>c the same in number in every shade but nm„ n • c °s. inclination. Or they may 
surface of paper .ball he the light’ „ r ’„ 5 If t,on 5 d “ thickness, so thut tii 
ness of the lines as the difference between radius •i^tw 1 ? pll ?“ tlon ’ alld ,Uc tbick 
fhe combination nf lmii. ... n ... ,, . oius and that to sine 
cue 
