DAY AND NIGHT THE SEASONS 77 



In regard to the other point, the longitude of the school, 

 as only a relatively small part of Great Britain lies to the east 

 of Greenwich, the probabilities are that the sun clock will be 

 slow by the school clock, i.e. noon will be later. If the difference in 

 longitude be considerable, it may be quite possible to show it 

 even by a rough school experiment. At Glasgow, for instance, 

 which is 4^ degrees west of Greenwich, noon by the sun 

 is (at times when the equation of time = o) seventeen minutes 

 later than noon by the clock, a difference which it should be easy 

 to show. 



The experiment of finding local noon may quite well be also 

 used to find a true north and south line, and may therefore be per- 

 formed with a little elaboration. 



Before beginning the experiment, be sure that the school clock, 

 or the watch to be used in the experiment, marks correct Greenwich 

 time, is as we say " right." Choose a fine day as near the I5th June 

 or i5th April as possible. Fix either a stout pin or a stick on a 

 level surface, taking care that the object is as nearly vertical as 

 possible, and that its shadow falls on a smooth surface. About 

 an hour before noon measure the length of the shadow, and make a 

 loop of thread or string, whose length is the exact length of the 

 shadow. The loop should be attached to the stick at one end, 

 and the other end should bear a piece of chalk, or something 

 with which a mark may be made. With this chalk describe a 

 semicircle, the length of the string being the radius, and the 

 stick the centre. Another method is to describe the semicircle 

 first, and then mark a point where the shadow touches it for the 

 first time. 



In either case, call the point where the shadow touches the 

 semicircle A, and the radius AC (see Fig. 4). If the first observa- 

 tion has been made about an hour before noon, it will be found 

 that the shadows grow shorter and shorter. Their length should 

 be marked at short intervals, and the points will be found to lie 

 within the semicircle. Mark the extremity of the shadow with 

 special care at the instant when the school clock strikes noon, 

 and go on making observations at intervals as the shadow creeps 

 outwards again until once again it touches the circumference of 

 the semicircle. Call this point B, and join AB. Bisect this 



