METEOROLOGY OF BEN NEVIS IN CLEAR AND IN FOGGY WEATHER. 469 



Co-ordinates of the Ben Nevis Observatory. — Latitude 56° 48' North ; Longtitude 

 5° 0' West of Greenwich. Elevation above the sea 4407 feet or 1343 metres. 



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

 showing Greenwich mean time. The mountain lies 5° of longtitude 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 observations. The first 

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

 tion should not be regulated by local apparent time, especially when, as at Ben Nevis, 

 hourly eye readings are made. When self-recording instruments are used, it as easy to 



Ben Nevis. — Dales when the Length of the Day is an exact number of Half-hours. 



Days from 













Days from 



vernal 



Diff. 



Date. 



Length of Day. 



Date. 



Diff. 



vernal 



equinox. 













equinox. 



93 



23 



June 21 



17 h. 32 m. 



21 June 





93 



70 



9 



May 29 



I7hrs. 



14 July 



23 



116 



61 



8 



„ 20 



W> 



23 „ 



9 



125 



53 



7 



,, 12 



16" 



31 „ 



8 



133 



46 



8 



» 5 



16J 



8 Aug. 



8 



141 



38 



6 



April 27 



15 



15 „ 



7 



. 148 , 



32 



7 



„ 21 



«* 



21 ,, 



6 



154 



25 



6 



„ 14 



14 



28 



- 7 



161 



19 



6 



„ 8 



131 



4 Sept. 



7 



168 



13 



7 



9 



13 



10 „ 



6 



174 



6 



6 



Mar. 26 



121 



16 „ 



6 



180 



365 



6 



„ 20 



12 



23 „ 



7 



187 



359 



6 



>, 14 



HI 



29 



6 



193 



353 



6 



,. 8 



11 



5 Oct. 



6 



199 



347 



7 



,. 2 



10J 



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 



» -1 



8J 



6 Nov. 



7 



231 



314 



7 



Jan. 28 



8 



13 „ 



1 



238 



307 



9 



„ 21 



n 



21 „ 



8 



246 



298 



22 



„ 12 



7 



30 „ 



9 



255 



276 



21 



Dec. 21 



6h. 28 m. 



21 Dec. 



21 



276 



