September io, 1908] 



NATURE 



445 



observing place, bounded by his horizon, appears to 

 lie at the top of the earth, and therefore parallel to 



Fk;. 5. — A model of the earth, -showing that when the poles He in the 

 plane of the true horizon, and therefore of the wooden horizon whicli 

 represents it. the horizon, represented by a wafer, of an observer 

 situated on the equator, is carried vertically up and down by the earth's 

 rotation ; this motion reflected causes the .-\pp:jrent up-and-down motion 

 of the stars as observed at the equator. 



FtG. 6. — In this case the axis is inclined to the wooden horizon, wni'.n in 

 parallel to the horizon of Britain when at the top of the globe. The 

 wafer represenlirig the horizon of Slonehenge is carried obliquely u-' 

 and dow-n in a direcuon parallel to the equator, so that the sun and 

 stars rise obliquely to the horizon. 



the wooden horizon ; let us therefore use two wafers 

 to represent local horizons, and place one on the 

 equator and the second on Britain. 



NO. 2028, VOL. 78] 



When we briii).f the equatorial w.ifcr Id the top of 

 the globe, where it lies parallel lo the wooden horizon, 

 we find th.it on rol.iliiig the globe it sweeps 

 down in a vertical plane. The wafer over Britain, 

 parallel to the wooden horizon when it is brought 

 to the top of the globe, when the globe is 

 rotated takes an inclined path to the horizon. This 

 happens because the axis, instead of lying in the 

 pl.-ine of the wooden horizon, is inclined to it. This 

 inclination of the axis varies with the latitude of the 

 place, and so the angle of inclination of the path of 

 the wafer to the wooden horizon varies with the 

 latitude. If we so arrange our model earth that 

 tlie inclination of the axis is the greatest possible 

 and the earth's equatorial plane lies in the plane of 

 the wooden horizon, it is obvious that the earth's 

 movement will only cause a wafer at the pole to 

 rotate; with this exception it will remain at rest, and 

 .'is there is no vertical motion to reflect, the stars will 

 neither rise nor set. 



Now the value of these little experiments depends 

 upon the already stated fact that I he apparent move- 

 ments of the heavenly bodies are brought about by 

 the real movements of the earth, and the experiments 

 show us that in reg.'ird to the horizon at any place 

 the true movement of the underlying earth, and 

 therefore the apparent movement of the overlying 

 heavens, is vastly different. 



.\t the equator an observer's horizon is being 

 whirled round in a vertical plane at the rate of 

 11)00 miles an hour; at the poles the horizon remains 

 p;:rallel to itself. In Britain we have a midway con- 

 dition. Correspondingly with these differences, at the 

 equator we have stars rising and setting vertically 

 and rapidly; in Britain stars rising and setting 

 obliquely and more slowly ; at the poles the stars 

 neither rise nor set. 



W'e may now return for a moment to Fig. 4, 

 which we have so far considered in relation to the 

 sphere of observation. It really enables us to study 

 as well the conditions of the celestial sphere for the 

 horizon N. E. S. W. of, let us say, Stonehenge in 

 lat. 51° N. P represents the position of the celestial 

 pole, and eqvv the inclination to the horizon of the 

 celestial equator for that latitude. The lines EQ and 

 .\s.M give the angle of slant as the sun or a star on the 

 equator or in a northern declination rises above the 

 horizon. 



Two or three technical terms which will be often 

 used afterwards may here again be referred to. pn 

 gives the height of the celestial pole, which is the 

 same as the latitude of the place, zp its zenith 

 distance ; it will be seen that these are complementan' 

 to each other, that is, together they maUe up 90'*. 

 s representing a star or the sun, PS is its polar 

 distance, as ks is its declination or distance from the 

 equator; it is seen that these again are complementary 

 to each other. The line sz represents its zenith dis- 

 tance. 



NORM.W LOCKYER. 



INTERXATIONAL CONGRESS ON TUBERCU- 

 LOSIS AT WASHINGTON. 



THE International Congress on Tuberculosis to be 

 held in Washington between .September 21 and 

 October 12 promises to be one of the most interesting 

 and important in the histor\- of these meetings. Pre- 

 sided over by the President of the United States, 

 assisted by Dr. Edward L. Trudeau, acting as 

 honorary- president. Dr. John S. Fulton, secretary- 

 general. Mr. Henry Phipps, of New York, as trea- 

 surer, an exceedingly strong national committee has 

 been brought together, and very complete arrange- 



