26 



ETON NATURE-STUDY 



of the day. At noon the shadow will have shortened as much as 



possible, and just as it begins to lengthen again, mark its position and 



draw lines upon the log so that on subsequent days the 



match box can be replaced in exactly the same position. 

 Careful comparisons will show that the shadow 



of the upright skewer will not fall in exactly the 



same place at the same time of day, 



and some alteration is needed before 



a sun-dial, which is at all reliable, s 



can be made. 



As a matter of fact the axis of 



the earth is tilted, and a long series 



of experiments would enable you 



to determine at what angle the 



skewer must be placed in order that FIGURE 29. 



the position of the shadow may be the same at the same times of day. 



By sliding out the match box from its 

 cover (see figure 29) the skewer may be 

 tilted as required. 



A simple geometrical figure will show 

 (see figure 30) that when the skewer or 

 the style of a sun-dial is parallel with the 

 axis of the earth, the angle made with the 

 surface of the latter must be the same 



FIGURE 30. . 



The angles marked with dots are by as that wlllch represents the latitude of 



Euclid, Book L, Proposition 29, equal the place where the observation is made 



to one another, and each together with Qr the sun _ dial fc ^ be erected. 



a black angle makes up a right angle ; 



therefore the blacked angles are equal. pUCB knowledge may be assumed and 



the latitude itself taken for granted, or this may be practically 

 determined in the following simple manner : 



