i68 



KNOWLEDGE & SCIENTIFIC NEWS. 



[August, 1904. 



The photographic self-recording instruments are very 

 costly, and need special buildings for their equipment, 

 and so can only be employed at first-class observatories. 

 By the introduction, during recent years, of the Richard 

 pattern of self-recording instruments, it is now possible 

 for observers to provide themselves at a moderate cost 

 with a small but useful thermograph, which can be 

 placed in the Stevenson screen, and which will give a 

 continuous record of the changes of temperature for a 

 whole week. 



Fig. 2.— Summer and Winter Diurnal Ranjie of Temperature. 



The following values show the annual diurnal range of 

 temperature at Greenwich, based on the 20 years' obser- 

 vations 1 849- 1 868, those with the + sign indicating that 

 the values were above, and those with the — below the 

 mean for the day ; — 



It will thus be seen that the temperature is at its 



minimum just before sunrise, and attains its maximum 

 between i and 2 o'clock in the afternoon. Fig. 2 gives 

 the summer and winter diurnal range of temperature. 



We might possibly imagine that the temperature would 

 progress uniformly day by day from its lowest point in 

 winter to its greatest height in summer. But such is by 

 no means the case. The variability of temperature is 

 very great. 



A striking instance occurred in January, 1901. At 

 Swarraton, near Alresford, Hants, the minimum on the 

 gth was as low as — i'9°, but the maximum on the loth 

 was as high as 49'2°, thus showing a range of 51-1" in two 

 days. In consequence of this great variability, it will be 

 understood that observations must extend over a large 

 number of years before the daily irregularities can be 

 smoothed out. Even 50 years is not long enough to 

 produce a smooth curve, as will be seen from Fig. 3, which 

 gives the mean temperature on every day at Greenwich 

 for the 50 years 1 841 -1890. This Fig. is reproduced 

 from a paper by Mr. W. Ellis, F.R.S., in the Quarterly 

 Journal of the Royal Meteorological Society, vol. xviii., p. 238. 



Some of the interruptions in the annual march of tem- 

 perature are very marked. These are not confined to the 

 south of England, but extend over a much wider area. 

 Dr. Buchan some time ago investigated the temperature 

 of Scotland for a number of years, and showed that the 

 following interruptions occur : — 



Six cold periods. 



( I July 12-15 



Three warm periods. -' 2 Aug. 12-15 



I 3 L)ec. 3- 9 



These interruptions are, no doubt, associated with cer- 

 tain types of weather which are accompanied by winds 

 from definite directions. Generally speaking, the effect 

 of the wind is as follows: — 



N. winds depress the temperature throughout the year. 



N.E. winds do the same, except in summer, when their 

 effect is small. 



Uc^. Jan. 



9 7 13 31 -32 Zt 



Feb 



March- 

 es 2J 



e 1! 30 



May 



JxjijLe 

 S n 23 



Jufy 



ft 23 



K 7i 



Sep. 



Octc 



3 IS 27 



Jiau 



Dec 



' 79 JJ 



F)?> 3* — Temperature on each day of the year. 1H41-1HQO. 



