Aug. 2i), 1884.] 



• KNOWLEDGE • 



175 



The effect, then, of winding an electromagnet evenly is 

 to produce an ordinary magnetic field as shown in Fig. C. 

 With the second mode of winding, i.e., coning the wire 

 towards the two ends of the iron, we obtain a held (Fig. 7) 

 similar to the one produced by the usual winding, with the 

 exception that the field between the poles is very weak. 

 With the two other modes of winding (Xos. 3 and 4) there 

 is a considerable concentration of force in the vicinity of the 

 covered end, while the uncovered end " seems to form a 

 long, weak pole," the difference between the two being that 

 with No. i as contrasted with No. 3 there is a greater con- 

 centration at the wound end, and that the opposite pole is 

 longer. The movement made by the " neutral zone " 

 towards the left hand is clearly indicated. 



The professors completed their experiments by ascer- 

 taining the weight necessary to detach the armature from 

 the covered end of each magnet, a constant or equal current 

 flowing through each coil. The result was that with : — 



1 43 ounces were required to detach the armature. 



2 5/ „ ,, ,, 



3 5i „ „ ,, 



■t 77 „ „ „ 



These results show, then, that the effect of coning the 

 wire is to produce a strong field near the pole, which, how- 

 ever (as may be gathered from Fig. 5), falls off rapidly as 

 the distance from the pole increases. It is also seen that 

 ill contact or close proximity with the magnet numbers, 

 2 and 3 are equal, and are stronger than No. 1, while the 

 effect produced by No. 4 is much greater than that pro- 

 duced by either of the others. The deductions to be drawn 

 are that with a given piece of iron, a given length of wire, 

 and a given current, at distances from the end of the 

 magnet very small compared with the length of the core, 

 the wire should be coiled up at the near end. At points 

 a little removed, equal, say, to one third the length of 

 the core, winding evenly over one half is advantageous, 

 while, for greater distance, uniform winding is the best 



THE EARTH'S SHAPE AXD MOTIONS. 



By Richard A Proctor. 



(Contimied from page 152.) 



CHAPTER III.— THE ANNUAL MOTION OF THE SUN 

 AND STARS. 



ALTHOUGH during a single day the sun's motion is 

 such as I have described it, yet it is impossible to 

 watch the sun many days without noticing that the place 

 of his rising and setting is continually changing, and also 

 the elevation which he attains when in the south. Sup- 

 posing our observations to commence in spring, we should 

 notice that the sun began to rise further and further to 

 the north of east, setting, of course, further and further 

 to the north of west. We should further see his mid-day 

 elevation gradually increasing. In about three months 

 these changes would attain their greatest effect, and at this 

 time we should find that the sun rose almost as far north 

 as north-east, and set almost as far north as north-west, 

 while at mid-day he attained an elevation of no less than 

 ()2°, instead of 3Si°, as at first. Then in the next three 

 months we should find these changes taking place in reverse 

 order, so that at the end of the three months the sun would 

 be rising nearly in the east and setting nearly in the west, 

 as at the beginning of the observations. After this the sun 

 would be found to rise towards the south of east, setting 

 towards the south of west, while his mid-day elevation 

 would continue to diminish. At the end of three more 



months these changes would produce their greatest effect, 

 when the sun would be rising nearly as far south as south- 

 east, and setting nearly as far south as south-west, attaining 

 a mid-day elevation of only 15^. Lastly, during the next 

 three months the sun would gradually return to the path 

 he had at the beginning of the observations. 



Year after year these circumstances are repeated with 

 the utmost regularity, so that the observer would find no 

 ditHculty in forming a table recording the height which the 

 sun would attain when due south on any day of the year. 

 This, at present, is all that we shall note on this point. 

 We shall shortly have to return to this part of our subject, 

 and by considering the annual changes of the sun's appa- 

 rent path more exactly — that is, in quantity and measure, 

 instead of in a general manner, we shall be enabled to form 

 an estimate of the real character of the sun's annual 

 motion relatively to our earth. At present, however, our 

 observer is supposed to be limiting his attention to those 

 observations which may enable him to determine the earth's 

 figure by travelling from the scene of his first researches. 

 He is, in fact, endeavouring to learn how the sun or the 

 stars would seem to move on any day of the year, as seen 

 from his first station, in order than when he goes to others 

 he may estimate the effect of change of place on these 

 motions, and so learn what is the shape of that surface 

 over which he is travelling. 



Our observer might notice also with special care where 

 the sun rises and sets on different days, were it not that 

 after some attention to this point, he would find that in 

 the immediate neighbourhood of the horizon, celestial 

 objects are not seen in their true places. He would quickly 

 see that this was an atmospheric effect, because he would 

 notice that even terrestrial objects, at a considerable dis- 

 tance, appear often to be disturbed or distorted. Presently 

 this peculiarity will need to be carefully considered. At 

 present, all that is necessary is that the obsei-ver should 

 avoid laying any stress on the observations of celestial 

 objects when very close to the horizon.* 



The result, then, to which our observer pays special 

 attention is the circumstance that on any given day in the 

 year the sun always attains a certain elevation when in tie 

 south. If he should find that when he travels to some 

 other place, the sun, on any particular day, does not attain 

 the elevation it would have had as seen from his first 

 station, he will have to explain that circumstance — he will, 

 in fact, have something to guide him to a true estimate of 

 the earth's figure, of which, at present, he is supposed to 

 know nothing, except that it has limits. 



In the meantime, and with a similar end in view, he 

 notices the annual changes in the apparent position of the 

 stars. This part of his work also leads to certain definite 

 and very interesting results. 



Night after night he sees the stars following the same 

 orderly movements which he had before detected. But 

 gradually he begins to notice, that, at any given hour of 

 the night, the aspect of the heavens is not the same as at 

 the same hour on the first night of observation. The stars 

 seem to have got further forward in their apparent rotation- 

 movement. 



Carefully noticing this change, he is led to the conclusion 

 that it is taking place in a regular manner. He can very 

 effectually test its rate and character by means of the 

 instrument figured in Chapter I. (Fig. 5), modified (as 

 described in Chapter II.) for sidereal observation. He 



* It may be necessary to remind the reader that the refractive 

 effects of the atmosphere slightly affect the apparent position of 

 every celestial object, but in this part of my subject I avoid aU. 

 reference to corrections so minute that the simple process of 

 observation I am describing would not suffice to detect them. 



