234 



♦ KNOWI.EDGE . 



[Sept. 19, 1884. 



two pieces mutually fit together. The betrinnerwill lie careful 

 to leave both pins and dovetails full large, in order that 

 the joint may be a tight one ; but they obviously must 

 not be loo large, or they will split the wood when driven 

 home. Assuming, then, that we have got a sufficiently 

 tight-fitting joint, the pins and hollows between the dove- 

 tails are brushed over with thin hot glue, and then the 

 parts are replaced, a mallet being used to drive the 

 pins home, as quickly as may be, and the whole affair put 

 aside for twelve hours or so, until the glue has set. 



When a box is being made, it is evident that there 

 will have to be lour sets of pins, with their corresponding 

 dovetails. 



The dovetail is a joint of such importance and in such 

 common use, that I recommend the amateur mechanic to 

 practise it on waste bits of deal for the sake of attaining 

 proficiency in it. 



THE EARTH'S SHAPE AND MOTIONS. 



By EicHAUD A. Proctor. 



CHAPTER IV. (i-onHnued). 



WHEN our observer has pushed his way as far south as 

 he can, at which time the southern pole of the heavens 

 will be nearly overhcid, lie will have completed his survey of 

 that particular section of his abode ; and having satisfied 

 liimself that it is a semicircle (wanting only two short arcs 

 at each end, which he has been prevented from traversing 

 by the difficulty which enormous icebergs have opposed to 

 his progress) he returns to his first station to commence 

 explorations in new directions. 



He first sets out towards the east — in other words, 

 keeping the pole of the heaveus continually on his left 

 hand. 



He finds now no change whatever in the aspect of the 

 heavens, and if he were not very watchful he might be led 

 to suspect that there was nothing to be gained by pursuing 

 his researches in this direction. 



But our observer is very watchful. He has set forth to 

 dtteimine the earth's figure, and he means to master the 

 problem, if it can possibly be done. 



As before, he has taken with him an accurate chro- 

 nometer, one which will not only enable him to measure 

 the rate at which the heavens rotate, when he is stationary 

 at any place upon his route, but to compare the aspect of 

 the heavens at any instant with the aspect which they 

 present at the place he started from. In other words, he 

 has a chronometfr which will enable him to know exactly 

 how many hours and minutes have passed since he left 

 home, let the interval which has elapsed be long or short. 



Now, in his former journey his chronometer, or rather 

 this particular quality of his chronometer, did not serve 

 him much. He found no change either in the uniform 

 character of the celestial motions, or in their rate ; and, 

 further, he found that on any day a star reached the 

 highest point of its path at exactly the same time as it 

 would have done when watched from his first station. 



But in this second journey this is not the case. When 

 he is stationed at any place the heavens rotate precisely 

 at the same rate as before ; but, while he is travelling, 

 the heavens seem to rotate faster. In other words, 

 according to the distance he travels towards the east he 

 finds the heavens farther advanced than he knows they 

 appear to persons at his oiiginal station. For example, 

 when he has travelled about 4.30 miles towards the east, 

 he finds that a particular star reaches its highest point 



in the north about forty minutes sooner than as seen by 

 his friends at home. 



It is clear that this was to a certain extent to have 

 been expected. As he is travelling towards the region 

 from beyond which the stars seem to rise, he might 

 naturally expect to see them rise sooner. But as he has 

 already convinced himself that the stars are at a distance 

 incomparably exceeding the distances he has travelled, he 

 knows that by travelling in a straight line he could not 

 produces such changes as he observes. 



For example, suppose that A (Fig. 3) is his first atation, 

 W A E the east-and-west line, S A N a part of the earth's 

 circular section in a noith and south direction. If at A, 

 a star is in the south, as at s, at R the star will appear 

 towards the west of south, or advanced towards its 

 setting, or in direction B s. But as s has been shown to 

 be very far off indeed, the angle A s B would be very 

 small, whereas it really amounts to about 40 minutes' 

 motion ; that is, to about 10 degrees. 



It is clear that if his second journey is like the first, 

 along a circular path, the observed change will be readily 

 explicable, because his horizon would then be continually 

 changing, and instead of having to assume a change 

 of place in the star, he could explain its observed change 

 of direction, as due to a change in the direction of the 

 north-and-south line, by which he estimates the star's 



Fig. 3. 



position. The change is one which it would be by no 

 means easy to illustrate satisfactorily, because in reality 

 the horizon plane is shifting in a rather complex manner. 

 But the attentive reader cannot fail, I think, to understand 

 the accompanying illustration. 



Fig. 4. 



Let A. Fig. 4, be the first station of the observer, SAN 

 the north-and-south line, P P' the direction of the earth's 



