Simple Way for determining the Time of Noon. 1/3 



limb of the sun's image crosses those semicircles, and there 

 will be several curves included between those points, through 

 the middle of which a right line being drawn from the centre 

 of the semicircles is a meridian line. After the line has been 

 drawn in this manner, it must be examined hy succeeding 

 observations, and corrected if necessary, by which means 

 a line may be drawn exceedingly near the true meridian, as 

 appears from my former paper on this subject *. 



Practical Observations. 



1. The metal plate, which may be about one-fifth of an 

 inch thick, must be placed parallel to the axis of the world, 

 that the sun's ravs may pass perpendicularly through it when 

 he is in the equator. 



£?. The aperture need not be more than one-fifth of an 

 inch in diameter, if it be countersunk on both sides, to admit 

 the sun's rays to flow through it at the distance of three or 

 four hours before and after noon. 



3. If the surface of the floor on which the observations 

 are to be taken for finding the 'meridian be not sufficiently 

 even, the floor may be covered with new boards, taking the 

 greatest care that they are laid down perfectly horizontal 

 from east to west. After the line has been correctly drawn, 

 and the north and south ends of it marked upon the walls of 

 the room, the boards may be taken away and others laid 

 down to draw the lines upon. 



A meridian line, upon a small scale, but sufficiently cor- 

 rect for regulating clocks and watches, may be had by the 

 following method : 



Let a stone with an even surface, about thtee feet long 

 and two broad, be fixed horizontally upon. a brick or stone 

 pillar at a convenient height for observation, with one of 

 its ends facing the south. Near the middle of tfus end of 

 the stone fix a anomon, in a direction perpendicular to the 

 horizon. This gnomon, which should be made of a strong 

 bar of metal, must have a small aperture made through it, 

 for the sun's rays to flow through early in the morning and 

 late in the evening;. From that point, as a centre, which is 



• Philosophical >'.gazin<\ ml. xxlv. p. 28P. 



di recti* 



