220 J. HOPKLNSON — THE CLIMATE OF WATFOED. 



can be calculated approximately, however, from the observed value 

 for such a period as ten years, if we know the mean rainfall over 

 the surrounding district, or even the mean at a single not far- 

 distant station, by comparing the mean for the shorter period with 

 that for the longer one, for we may reasonably infer that the rela- 

 tion subsisting in (say) ten years between the rainfall at two or 

 more adjacent stations, will be approximately maintained for forty 

 or fifty years, should no physical change affecting climate, such as 

 alteration of level or clearing of forests, have taken place. 



After temperature and rainfall, the sequence of phenomena in 

 relative importance, as indicating the nature of the climate of any 

 place, is probably humidity, cloud, and wind. Atmospheric pres- 

 sure can scarcely be considered an element of climate, but in deter- 

 mining the character of other meteorological phenomena it is of 

 paramount importance, and it will therefore be discussed first. 

 For a similar reason — the cause preceding the effect — humidity, 

 cloud, and wind will be discussed before rainfall. 



But before proceeding to the consideration of the general results 

 arrived at from the observation of these phenomena, it is necessary 

 to give some information as to the localities where the observations 

 have been made, the instruments used, and the method of observa- 

 tion and reduction. 



The observations were commenced at Holly Bank, Watford, on 

 the 1st of March, 1876. At the end of September in the following 

 year I removed to Wansford House, "Watford, where they were 

 carried on from that time until the end of the year 1886, when 

 I removed to St. Albans. Observations have thus been taken at 

 Wansford House for nine years and three months. Ten years is, 

 however, a period to be preferred to nine, for, besides having 

 the advantage of being of longer duration, it is much more con- 

 venient to work with, and moreover it is the usual period, between 

 that for one year and for a quarter of a century, for which results 

 of meteorological observations are published. As the localities are 

 only about half a mile apart, the observations of as many months 

 at one of them as of years at the other, cannot, I think, insomuch 

 as the slight difference between the two situations is concerned, 

 materially affect the general results of the observations at Wansford 

 House. I have, therefore, included the records of observations for 

 nine months at Holly Bank with those for nine years and three 

 months at Wansford House. A description of each locality having 

 already appeared in the annual reports of my observations con- 

 tributed to the Society, with full particulars of the instruments 

 used, etc., it will suffice to give here only a brief summary of 

 information most of which may be found in greater detail in 

 previous volumes of our Transactions.* At first my observatory 

 was a " Second-order" Station of the Royal Meteorological Society 

 (obs. taken at 9 a.m. and 9 p.m.), but for the greater portion of 

 the time it was a Climatological Station (obs. taken at 9 a.m. only). 



* 'Trans. Watford Nat. Hist. Soc.,' Vol. I. pp. 217-219; ib., Vol. II, 

 pp. 209-211 ; and 'Trans. Herts Nat. Hist. Soc.,' Vol. Ill, pp. 181, 182. 



