76 THE BOOK OF NATURE STUDY 



up to the subject, making the members of the class do some 

 actual constructive work which would interest every one, even 

 those whose interest in the theory of the matter was weak. 



In Simmons, and Richardson's Introduction to Practical 

 Geography there is a very full treatment of sundials, and some 

 of the simpler types of those described might be very well con- 

 structed. It should, however, be carefully noted that while as 

 shipwrecked sailors we could divide our day as satisfactorily by a 

 sun-clock as by a watch, as members of a civilised community we 

 have to note that our sun-clock, however carefully constructed, 

 does not coincide with the actual clock. The reason is, of course, 

 twofold. In the first place, clock noon in this country is, except 

 in Ireland, Greenwich noon, and the school may be considerably 

 west of Greenwich. Secondly, as clock noon is mean noon, the 

 correction necessitated by the earth's varying velocity (equation 

 of time) has to be applied. 



In the author's opinion it would be quite unsuitable, at the 

 Nature Study stage, to attempt to give a full explanation of the 

 term equation of time. The explanation demands considerable 

 power of abstraction, and cannot be made clear without diagrams 

 and apparatus. 



If the teacher finds it necessary, and especially if the school is 

 situated in a port and among seafaring folk who take a natural 

 interest in navigation, there seems no objection, when attempting 

 to find noon by a shadow-clock, to applying the necessary correc- 

 tion from a Nautical Almanack, without explanation, as many a 

 sailor would apply it, leaving the explanation to come later. 



If it is not desired to mention the subject at all, then the attempt 

 to find the true noon by the sun should be made at a time of 

 year when the correction is so small that it may be neglected in 

 the rough experiments performed. For this reason it is well to 

 make the experiment towards the middle of June, the middle of 

 April, or the beginning of September. At these times the correc- 

 tion is a matter of seconds only for some days. 1 June is an espe- 

 cially suitable month, on account of the probability of sunshine. 



1 It should be noted that the sun-clock and mean time coincide on four days of the 

 year, i.e. on four days the equation of time = o. These days are i$th April, i4th June, 

 ist September, 25th December. - 



