INTRODUCTION. XXXI 



an increase of one degree of latitude, and an eleyation of 100 yards 

 also causes a decrease of one degree of temperature. Hence, if T be 

 the mean temperature of Greenwich (alt. 155 ft., lat. 51-5°) at the 

 sea level, T', the mean temperature of anyplace not very remote from 

 Greenwich, in England for example, may be obtained approximately 

 from the following formula : — 



T^=T+(51-o°- 0x09- Ax -00.345, 



where I is the latitude of the place and h its height above the sea 

 level. 



The mean temperature of Middlesex will be only subject to a vari- 

 ation of about 0o° from range of latitude ; and as this takes place 

 about equally on either side of the parallel of Greenwich, the mean 

 temperature of that place will represent with great accuracy the mean 

 temperature of the coimty. The mean temperature of Greenwich 

 has been determined from twenty-eight years' observations, ending 

 18G8, to be 49-4°. At Chiswick, from observations made in the 

 garden of the Royal Horticultural Society for seventeen years ending 

 1842, the mean temperature was found to be 49-88° (Daniell's 

 Meteorology, vol. ii. p. 364). From observations made at Tottenham 

 and Stratford from 1817 to 1830, it was found by Howard to be 

 49-651° {Climate of London, vol. i. p. 40). 



The mean temperature of London itself -was found by Howard to 

 be somewhat higher than that of the adjoining country from, as he 

 supposed, 'the effect of the population and tires' {Ibid. p. 236). 

 The difference is, however, now known to be due to the proximity 

 of the Thames. In discussing the observations made at the rooms 

 of the Royal Society at Somerset Hou.'se {Phil. Trans. 1850), 

 Mr. Glaisher found that ' upon the whole year the excess of tempera- 

 ture at Somerset House was 1-2°.' He considers that ' one part of 

 these differences is owing to the difference of elevation, and will 

 probably amount to about 0-3° ; the greater part of the remaining 

 difference is most probably owing to the vicinity of the water of the 

 Thames, whose temperature during the night hours at all seasons of 

 the year is several degrees higher than that of the air. Those parts 

 of London situated near the River Thames are somewhat warmer 

 upon the whole year than the country ; but those parts of London 

 which are situated at some distance from the river do not enjoy 

 higher temperatures than those due to their latitudes ' (p. 580). For 

 example, ' no certain difference exists between the temperatures of the 

 air at Greenwich and at St. John's Wood ' (p. 578), The temperature 

 of Fleet Street is 0-7° higher than Greenwich (p. 578 ; see also Drew's 



