Dec. 27, 1883] 



NA TURE 



=05 



will be asked, How are these observations made, and how is it 

 known when the star is in the same position when the second 

 observation is made ? 



For this purpose a transit instiument is used (see Fig. 30). 

 This differs from an ordinary telescope, being so mounted as to 

 move only up and down, and is armed not with simple cross 

 wires, but with an odd number of parallel and equidistant 

 vertical wires crossed by a single horizontal wire. It is also 

 usually pro. ded with a circle to give declination. If from any 

 part of the earth an observation be made on any particular star 

 on one day, and then another observation made on the same star 

 when it is in the same ]3osition the next day, as has been said, 

 the interval between the two observalions must be the time 

 taken by the earth to move round once. 



By having such an arrangement as exists in the transit instru- 



FiG. 12.— Showing that the true horizon of a pole is the equator. 



ment, by which it can swing in the plane which coincides with the 

 axis on which the earth turns, any star may be chosen for the 

 observation. Suppose, for instance, the instrument be pointed to 

 the north pole star, then, in consequence of the tremendous 

 distance of the stars, the axis of the telescope is practically 

 coincident with the axis of the earth. But suppose another star 

 to be observed, it will be quite clear that we may make the 

 observation on it, or any other star we choo=e. When the 

 instrument is upright it points to the zenith. A star in the 

 zenith may therefore be selected for the observation. 

 '- It is observed when crossing the central wire of the instrument 

 one day, and noted again when it crosses that wire on the 

 succeeding day. But the observer does not limit his observation 

 to the one central wire, in order to ascertain when the star is in 

 the centre of the field. If he did so, he might miss his observa- 



Fig. 33.— Showing that the poles lie in the horizon at the equator. 



tion. That is why the simple cross wires have been replaced by 

 a system of wires (see Fig. 31). As the star crosses the field of 

 view, the observer, listening to the beats of the clock alongside, 

 notes the time when it crosses each of the wires, and takes the 

 mean of these observations, thus attaining to a much greater 

 accuracy than if he had merely observed the transit over the 

 central wire. With an ordinary clock it is found that a period, 

 less by a few moments thin twenty-four hours, elapses between 

 two successive transits. 



In order to get an absolutely perfect measure of time, the 

 clock may be so rated that it .should not be any indetermiuate 

 number of hours, minutes, and seconds but twenty-four hours 

 exactly between the two transits of that star. With a clock thus 

 arranged, the time at which a star crossed the central wire of the 



transit instrument would really give a most perfect method of 

 determining that star's place in the heavens, because, if the 

 earth's rotation is an equable one and takes place in a period 

 which we choose to call twenty-four hours, then two stars 180° 

 apart will be observed twelve hours after one another, four stars 

 90" apart will be observed six hours apart, and so on ; and clocks 

 like this, regulated to this star time, exist in our observatories, 

 being called sidereal clocks, because the time they give, which 

 is not quite familiar to everybody, is called sidereal time. 



Now let us consider our position on the earth with regard to 

 the stars. This is a very interesting part of our subject, not 

 only in its scientific aspect, but from the point of view of its 

 usefulness, whether we wish to study the stars or define places 

 on the earth's sm'face, the latter matter, however, being so 

 intimately connected with astronomy proper that it is impossible 

 to talk about the one without talking about the other. 



Since we divide all circles into 360", the circumference of the 

 earth maybe so divided, and the method in use of defining positions 

 on the earth is to say of a place that its latitude is so much and 

 its longitude is so much. Latitude begins at the equator with o", 

 and terminates at the poles with 90°, being north latitude in the 

 one case, and south latitude in the other. In the case of 

 longitude, there is no such simple starting point, for whilst lati- 

 tude is counted from the equator by everybody all over the world, 

 longitude may commence at any point. In England we count 

 longitude from the meridian of Greenwich. When the transit 

 instrument at Greenwich is swept from the north point through 

 the zenith to the south point it describes a half circle, which is 

 called the meridian of Greenwich. 



mid-latitude, 



That is one point. Another point is this. Suppose the 

 instrument to be set up not at Greenwich but at the north pole. 

 Then the true horizon of the observer will be along the equator. 

 Remove the instrument to the equator, and the true horizon will 

 cut the poles. At a place in mid-latitude the true horizon 

 would cut neither the pole nor the equator, but would be 

 inclined to both (see Figs. 32, 33, and 34). 



Then comes the important relationship between the latitude 

 of the place and the altitude of the pole star above its horizon ; 

 that the number of degrees this star — be it north or south — is 

 above the horizon of the observer will be the number of degrees 

 of north or south latitude of the place where the observation is 

 made. A place therefore in 10° N. lat. will (roughly) have the 

 north pole star at a height of 10° above its horizon. 



So much for this part of our subject. Let us now leave it, 

 because, interesting as it is, it refers to a branch of astronomy 

 with which at present we have less to do than with the m >i'e 

 physical one ; but it was well that we should pause for a few 

 moments to note the tremendous importance to mankind of that 

 particular movement of the earth which we have been con- 

 sidering. J. Norman Lockyer 

 ( To be contimtfd. ) 



PROBABLE NA TURE OF THE INTERNAL 



SYMMETRY OF CRYSTALS^ 



'X'HE theory of the modification of crystal angles, just olTered 



in dealing with quartz, is manifestly applicable to all crystals 



notl[of the cubic system, and it is submitted that for every such 



' Cor.tinued from p. 188. 



