I.S6 



XATURE 



[October 24. 19 1 8 



transit. The line joining tbesi points is thi graph 

 required, hour angle tor any position beinj 



h 1 and zenith distam i on margin. 

 Hen polar distance i / 



30 . 

 I in refore (p el for left ma = 6o°. 



{p+c) ,. right .. i-o c . 



The graph being draw n acci 



on at foot of chart, we have zenith dis 

 73 31' on margin. Whin the sun is setting the zenith 



is 90 , and the I ilso 90 , 01 



6 hours. To find the hour angle at thi end of twilight 

 it is, u hi n the 31 8 — we 



have to draw the parallel for 90°+i8 . or 108 . The 

 graph intersects this in the poinl (a), which would be 

 found on me espi md approximately 



« iih 8h. 33m. p.m. 



I imuth and Polar Di I 



[ntei 1 hanging pol: nd zi nith dis- 



edun will be \<t\ much as before. 



place on the equator find the 

 azimuth of bodies of di clination 14' 29' N., o°, 



j 19' S., Mi' altitude in each cas \ 60°. 



Rule. On left-hand margin mark [z~c), and on 

 right-hand margin (z+c). Join these points, and 

 azimuth for any position is read off on base, and polar 



(1) ieo' 



(•875) IS8"25' 



(75) 120* 



(■62S.) 104*23* 



f5) SO' 



(-375) 7S*;:' 



( 25) 60* 



(125) 41*25' 



(0) o* 



4125 6w' 75 il SO 10*29 120 lie'; 



Here (c-z)=90°-3o°=6o°, (c+c) = 9 o + 3o = u.r. 

 For declination 14° 29' N., we have polar distance 

 75 31', and azimuth N.6o°W. ; for declination o°, 

 polar distance is 90 , and azimuth X. oo : \\". ; for 

 declination 14° 2q' S., polar distance is 104 29', and 

 azimuth X. 120° W. or S. 60 W. 



The following is an example of the converse case in 

 which decliri ttion is obtained from azimuth :- -In lati- 

 tude 45 N. find the declination of a body which passi s 

 thi prin il al an altitude equal to the latitude 



of place. For ( -c) we have the value zero, so thai 

 the graph through the origin, while (z + c) 90 



If the I- • ■ 1 polai distant , 



so thai di clin; 1 is -o c \. If the azimuth is 60 . 

 it is also evident from the diagram that polar distanci 

 is 41 25', and declination 48' 35' X. 



The deduction of declination from observed altitude 

 and approximate azimuth is of valui at si a to identify 

 an unknown 



The most obvious use of the diagram is to supply 



ceedingl} simpli method for azimuth. In 



the diagram can be I facility for 



hour angle. But in the latter problem much gri 



accurac; is required than in thi i hi hi diagram 



XO. 2556, VOL. I02] 



mi 1 ssar » ould ha\ e to be upon too la 



be avail. Mi for ordinary use at sea. [l is quite pos- 

 sible, however, thai another kind of navigation may 

 matti 1 of daily expei ieni . viz. the 



long-distance navigation of the air, and thai in this 

 form ol navigation, which will undoub 

 many features peculiar to itself, thi diagram mav 

 rail} not only for azimuth purposes, but also 

 ■ ol houi .1 

 In the a ..til- ..I the inventor of the diagram: — 

 " rhi easibilit" thus disclosed ol framing .1 nautical 



1 which ..Il requirements will 

 by a singli trigonometrical table, lik. h 



-, No. 4i in the An 

 inves d thi subjeel with interest from the point of 

 view i't aerial navigation, because this formula, if 

 . al in] ni, might pro- 

 \ ; i'n- ti aerial navigator with the equivalent of a 

 v.. Inn : 1 hi M.1111 ical .i-iii mom\ in a form - . ■ 

 to fulfil the instant ... 1 .1- ii flight." H. B. G. 



EXPERIMENTAL STl DIES OF THE ME- 

 CHAXICAL PROPERTIES OF MATERIALS* 

 r y* HE general purpose of 1 



■*- is to distinguish between the tit and unfit, 



and unsuitable materials foi the various 

 requirements of the structural and 1 work 



of the world. The special objeel of the engineer in 

 testing materials is to obtain .1 rational basis fo 

 portioning structun s and machines so that the} may 

 sustain the straining actions to which thej are sub- 

 jected without fracture or prejudicial deformation, and 

 at the same time without waste of material. Nor 

 is then any finality in such testing, for nfw allo\s, 

 new heat treatments, new conditions of use are 

 always making fresh investigation necessary. In the 

 next place, the mechanical properties of materials 

 desired and assumed in designing are embodied in 

 specifii 1 i.uis. Thence arises a second occasion for 

 experiment, rests of reception or inspection tests 

 in 1 1 ssari to determine whether material supplied 

 reaches the required standard. With thi widi 

 of tin sources of supply, an engineer can no 1 

 depend merely on the reputation of the seller, but 

 must make his own tests. 



£iir/y Researches. 

 Then are two methods, -aid Roger Bacon in 

 thirteenth century, by which we acquire knowledgi — 

 argument and experiment; and he proved the fertility 

 of the method of experiment in contrast with thi 

 1. .ni. 11 dialectics of his time. Hut it was sorrn 

 turies later before anything was done to ascertain by 

 experiment the data required by the engineer in using 

 materials of construction. Yet there is no su 

 of greater importance to engineers, or of more intel- 

 lectual interest, than the stud\ of tin mechanical 

 ies of materials which fit them for use in con- 

 struction. Nor is there one which n con- 

 cerns tin general public who depend on the pri 

 of machinery and travel on rail 



The earliest known experiments on the strength of 

 material- were mad. by G in 1638, an.'. 



Ftei Muschenlii in k. of Levden, made many 

 list- on a small scale, -..ni. <.f which are quoted in 

 Barlow'- •Strength of Materials." Galileo knew 

 nothing of elasticity , but he d 



I started ry into the sir. rig 



ng a solution partly right, partly wr. 



t From ihe Thomas Hawltstey Lecum- delivered l.eforc the Institution of 

 Me. I. . ii, ,1 Enprineewon 1 ' 1 W. Cawthorne Unwin, F.R.s 



Histoirede ' V ... . mie <le* Science--, 

 tio ad coharrentiam corporum tirmorum," 1729; B; 

 Materials," 1863 



