150 



♦ KNOWLEDGE ♦ 



[Ado. 21, 1885. 



Let us see ]iow this is brought about, not iudoed 

 inquiriuu- iuto the iiroccsscs by which at the Greenwich 

 ObsLTvatory the elements of safety are obtained, but 

 considering the method bj- which a seaman makes use of 

 those elements. 



In the measures heretofore considered, the captain of a 

 ship in reality relies on terrestrial measurements. He 

 reasons that, having been on such and such a day in a 

 given place, and having in the interval sailed so many 

 miles in such and such directions, he must at the moment 

 be in such and such a place. This is called navigation. 

 In the processes next to be considered, which constitute 

 a part of the science of nautical astronomy, the seaman 

 trusts to celestial observations independent of all terrestrial 

 measurements. 



The jioints to be determiiu.a l,y tin \,lv;l^x■^ are his 

 latitude and longitude. Tlir l;i"iiiu.|.' i-,' t lie distance 

 north or south of the equatnr, ;iinl i.-, i;ii;iMu-ed always 

 from the equator in degrees, tlic diritaiicf from equator 

 to pole being divided into ninety equal parts, each of 

 which is a degree.* The longitude is the distance east 

 or west of Greenwich (in English usage, and now 

 most other nations employ the same starting-point for 

 measuring' longitudes from). Longitude is not measured 

 in miles, but in degrees. The way of measuring is 

 not very readily explained without a globe, but may 

 be thus indicated : — Suppose a circle to run completely 

 round the earth, through Greenwich and both the poles ; 

 now if this cii'cle be supposed free to turn upon the 

 polar axis, or on the poles as pivots, and the half which 

 crosses Greenwich be carried (the nearest way round) 

 till it crosses some other station, then the arc through 

 which it is can-ied is called the longitude of the station, 

 and the longitude is easterly or westerly according as 

 this half-circle has to be shifted towards the east or west. 

 A complete half -turn is 180 degrees, and by taking such 

 a half-turn either eastwardly or westwardly, the whole 

 surface of the earth is included. Points which are 180 

 degrees east of Greenwich are thus also 180 degrees 

 west of Greenwich. 



So much is premised in the way of explanation to 

 make the present paper comislete ; but ten minutes' 

 inspection of an ordinary terrestrial globe will show the 

 true meaning of latitude and longitude more clearly (to 

 those who happen to have forgotten what they learned 

 at school on these points) than any verbal description. 



Now it is sufficiently easy for a sea-captain in fine 

 weather to determine his latitude. For places in different 

 latitudes have different celestial scenery, if one may so 

 describe the aspect of the stellar heavens by night and 

 the apparent path of the sun by day. The height of the 

 pole-star above the horizon, for instance, at once indicates 

 the latitude very closely, and would indicate the latitude 

 exactly if the pole-star were exactly at the pole instead 

 of being merely close to it. But the height of any 

 known star when due south also gives the latitude. For 

 at every place in a given latitude, a star rises to a given 

 greatest height when due south; if we travel farther 

 south the star will be higher when due south ; if we 



with the utilitarian labours for which our national Observatory was 



* Thronghont this explanation all minnter details are neglected. 

 In reality, in conseqnence of the flattening of the earth's globe, the 

 degrees of latitude are not equal, being larger the farther we go 

 from the equator. Moreover, strictly speaking, it is incorrect to 

 speak of distances being divided into degrees, or to say that a 

 degree of latitude or longitude contains so many miles ; yet it is so 

 exceedingly inconvenient to employ any other way of speaking in 

 popular description that 1 trust astronomers or mathematicians 

 who may read this article will forgive the solecism. 



travel farther north it will be lower ; and thus its ob- 

 served height shows just how far north of the equator 

 any northerly station is ; while if the traveller is in the 

 southern hemisphere corresponding observations show 

 how far to the south of the equator he is. 



But commonly the seaman trusts to observation of the 

 sun to give him his latitude. The observation is made 

 at noon, when the sun is highest above the horizon. 

 The actual height is determined by means of the 

 instrument called the sextant. This instrument need 

 not be here described ; but thus much may be mentioned 

 to explain that process of taking the sun's meridian 

 altitude which no d.iubt eveiy one has witnessed 

 who has taken a long seu-jdurney. The sextant is sa 

 devised that the observer can see two objects at once, 

 one directly and the other after reflection of its light ; 

 and the amount by which he has to move a certain 

 bar carrying the reflecting arrangement, in order to 

 bring the two objects into view in the same direction, 

 shows him the real divergence of lines drawn from his 

 eye to the two objects. To take the sun's altitude then 

 with this instrument, the observer takes the sim as one 

 object and the horizon directly below the sun as the 

 other : he brings them into view together, and then 

 looking at the sextant to see how much he has had to 

 move the swinging arm which carries the reflecting 

 glasses, he learns how high the sun is. This being done 

 at noon, with proper arrangements to ensure that the 

 greatest height then reached by the sun is observed, at 

 once indicates the latitude of the observer. Suppose, for 

 example, he finds the sun to be forty degrees above the 

 horizon, and the "Nautical Almanac" tells him that at 

 the time the sun is ten degrees north of the celestial 

 equator, then he knows that the celestial equator is thirty 

 degrees above the southern horizon. The pole of the 

 heavens is therefore sixty degrees above the northern 

 horizon, and the voyager is in sixty degrees north lati- 

 tude. Of course, in all ordinary cases the number of 

 degrees is not exact, as I have here for simplicity sup- 

 posed, and there are some niceties of observation which 

 woiild have to be taken into account in real work. But 

 the principle of the method is sufficiently indicated by 

 what has been said, and no useful purpose could be served 

 by considering minutire. 



(To he continued.) 



THOUGHT AND LANGUAGE. 



By Ada S. Ballin. 



thew.ii 



idr:: 



A c 



eof 



this kind is mentioned by Winslow.* Dr. Bast inn t eon 

 siders that, "roughly speaking, inability to reeiiU iiiniies, 

 or the miscalling of persons, places, or thin- s, wmild bo 

 defects o-oinq- with injuries to, or altered states of, percep- 

 tive centres,' and might exist with comiiaratively slight 

 impairment of iutelleetual activity; whilst, on the other 

 hand, the extreme forms of amnesia, in which wholly 



