EECEEATIVE SCIENCE. 



ass 



the sun to shine, at his southing, through the 

 hole outside, on the plumb-line, throughout 

 the year. Having fixed the plumb-line ap- 

 proximately, by observing the sun when he 

 is about south — of course not forgetting the 

 equation of time — the next thing is to get a 

 small wooden pillar, the height of the room, 

 with a vertical slit in it, or a set of holes, to 

 enable the observer to see a second plumb- 

 line, also in the plane of the meridian, which 

 he should attach to the ceiling some feet 

 further from the window than the principal 

 one. The object of the^econd plumb-line is 

 to enable the observer to avoid parallax be- 

 tween the plumb-line and the slide (referred 

 to hereafter), by enabling him to keep the 

 eye in the plane of the meridian when setting 

 the slide. 



During the observation, it must be re- 

 membered that the observer sits or stands 

 with his hach to the sun. A little wooden 

 box must now be used, open on two opposite 

 sides, to enable it to slide up and down the 

 pillar, to suit the sun's altitude at the diffe- 

 rent times of the year. The box should have 

 a " set-screw" on one side, aad a dovetailed 

 groove on the south side, in which is to slide 

 horizontally a piece of wood on which is 

 pasted white paper to receive the beam of 

 light coming through the slit, and also for 

 the graduations by which the central observa- 

 tion is to be checked. 



The pillar must be so placed to the north 

 of the principal plumb-line — the one close to 

 the pillar — that the plumb-line may hang 

 about 1-lOth of an inch to the south of the 

 outside vertical face of the sliding piece of 

 wood. Two vertical lines should now be 

 . drawn on the prepared face of the slide, at 

 a distance from one end of it say of about 

 one-third of the length of the slide, which 

 should be six or eight inches long, the verti- 

 cal lines to be sufficiently far apart to suit the 

 average width of the beam of light coming 

 through the slit in the metal outside, at the 

 different periods of the year. The space in- 



cluded between these two lines should be 

 bisected by a third line parallel to the others, 

 the use of the last being for the central ob- 

 servation of the passage of the beam of light 

 across the meridian line, or plane. 



Other vertical lines may now be drawn on 

 one side only of the central space, at distances 

 to suit the observer's convenience, the use of 

 them being to enable him to get pairs of ob- 

 servations, composed of those before and 

 after the sun's passing the centre ; by taking 

 the means of these, after reversing the slide, 

 he will be able to check the central one.* 



We will now suppose the observer to hava 

 completed his arrangements for taking tha 

 observation. Having ascertained the position 

 which the beam of light will occupy, with the 

 given declination for the day, he stands or 

 sits with his back to the sun, having dark- 

 ened the room if he is desirous of getting a 

 better defined image of the sun. The slide 

 being arranged so as to move easily by the 

 hand, or by means of a screw (which would 

 be an improvement on the plan we have been 

 accustomed to adopt), the observer places tha 

 slide so that one of the vertical lines shall be 

 in the plane of the meridian, and he now 

 waits, watch in hand, till the beam of light 

 gets to the space marked out for it. 



The lines should be numbered 1, 2, 3, etc. 

 The instant when the beam of light occupies 

 the centre of the space alluded to being 

 noted, the slide is quickly pushed forward, 

 from left to right, till the next line is in the 

 plane of the meridian. So on till the sun ar- 

 rives at the meridian. The central observa- 

 tion is then taken. The slide is now reversed, 

 so that the very same lines are used after noon 

 as before noon ; and thus no error can arise 

 from graduation. By this method, which is 

 original, the near approach towards the truth 



* The amplication of the principle of pairs of ob- 

 servations to tha meridian lino, as well as many other 

 advantages which the line now described affordSj 

 should be placed to the credit of Mr. Bunt, the well- 

 known tide-computer, etc., who suggested many oris 

 ginal ideas in reference to it. 



