786 



MR J. Y. BUCHANAN ON THE 



Hence, out of the thirteen years we can make up four years in which the weather was 

 either continuously clear or continuously foggy in the sense of our specification of these 

 classes of weather. 



Time used in the Observatory. — The observations are made at every hour by a clock 

 showing Greenwich mean time. The mountain lies 5° of longitude west of Greenwich, 

 so that the local mean time of Ben Nevis is twenty minutes earlier than mean time 

 at Greenwich. With the varying values of the equation of time the local apparent 

 time varies from 3*5 to 34*5 minutes earlier than Greenwich mean time. In the first of 

 the accompanying tables (printed on the preceding page) is given, for a number of dates, 

 the number of minutes after Greenwich mean noon when it is local apparent noon at 

 Ben Nevis. It will be seen that at the beginning of November the sun crosses the 

 meridian at only 3 "5 minutes after noon by the clock ; while at the beginning of 

 February it does not do so till nearly thirty-five minutes after noon by the clock. 

 Between 15th January and 10th March, 1 p.m. is nearer true local noon than 12 o'clock 

 by the clock. 



The second Table gives the dates of days of lengths differing by half-an-hour, with 

 the number of days elapsed since the vernal equinox. 



Both of these Tables are useful in interpreting the meaning of the results. The first 

 naturally suggests the question whether at first-class observatories the hours of observa- 



Ben Nevis. — Dates tohen the Length of the Day is an exact number* of Half-hours. 



Days from 













Days from 



vernal 



Dilf. 



Date. 



Length of Day. 



Date. 



Diff. 



vernal 



equinox. 













equinox. 



93 



23 



June 21 



17 h. 32 m. 



21 June 





93 



70 



9 



May 29 



17 hrs. 



14 July 



23 



116 



61 



8 



„ 20 



161 



23 „ 



9 



125 



53 



7 



„ 12 



16 



31 „ 



8 



133 



46 



8 



„ 5 



151 



8 Aug. 



8 



141 



38 



6 



April 27 



15 



15 „ 



7 



148 



32 



7 



., 21 



1*4 



21 „ 



6 



154' 



25 



6 



„ 14 



14 



28 „ 



7 



161 



19 



6 



i, 8 



13* 



4 Sept. 



7 



168 



13 



7 



2 



13 



10 „ 



6 



174 



6 



6 



Mar. 26 



12* 



16 „ 



6 



180 



365 



6 



„ 20 



12' 



23 „ 



7 



187 



359 



6 



» H 



iH 



29 „ 



6 



193 



353 



6 



.. 8 



11 



5 Oct. 



6 



199 



347 



7 



2 



10£ 



11 „ 



6 



205 



340 



6 



Feb. 23 



10 



18 „ 



7 



212 



334 



6 



„ 17 



H 



24 „ 



6 



218 



328 



7 



„ 11 



9 



30 „ 



6 



224 



321 



7 



„ 4 



8* 



6 Nov. 



7 



231 



314 



7 



Jan. 28 



8 



13 „ 



7 



238 



307 



9 



„ 21 



n 



21 „ 



8 



246 



298 



22 



„ 12 



7 



30 „ 



9 



255 



276 



21 



Dec. 21 



6 h. 28 m. 



21 Dec. 



21 



276 



