ON THE RAINFALL IN THE BRITISH ISLES. 451 



complete for England and for the ten years 1850-59 inclusive. The resiilts 

 are so striking and seem likely to have snch an important bearing on ques- 

 tions of water supply, and (probably to a less extent) on agricultural matters, 

 that we think it would be wrong to suppress them because we hope to report 

 fully on the subject next year, and that we shall best discharge our duty by 

 reporting the facts which have been ascertained, but prefixing the caution 

 that although these deductions are based on daily observations for ten years 

 at each of ninety stations, yet that this apparently fu-m basis is by no means 

 unassailable ; the laws that hold good in one ten years may not be so mark- 

 edly confirmed by other ten years, and those which hold south of the Tweed 

 may be reversed or modified in the sister coimtries. Table II, indicates the 

 results at present obtained. 



8. Approximate Determination of the Height of Rain-gaw/e Stations above 

 Sea-level. — There having been very many stations (perhaps 500 out of a 

 total of 1500) the altitude of which above the mean level of the sea was un- 

 known, considerable attention has been given to the determination of this 

 important element. Before noticing the steps we have recently taken, it 

 may be well to state briefiy what is meant by the apparently simple term 

 " height above mean sea-level," and to what extent this information is ob- 

 tainable. " Mean sea-level," otherwise called " Ordnance Datum," is the 

 mean height of the sea as recorded by the tide-gauge erected at Liverpool by 

 the Ordnance Survey Department, and it is the zero from which all their 

 altitudes are measured. The altitudes determined by the Ordnance Depart- 

 ment have been published in two forms — (1) in works entitled " Abstracts of 

 Spirit Levelling in England and Wales, Scotland and Ireland," wherein, as 

 the title imphes, are given the heights of certain marks, called Benchmarks, 

 (7I\) (^ut on milestones and other permanent objects along some of the 

 principal roads in the British Isles ; (2) in the maps on the 6 -inch (and now 

 on some of the 1-inch) scale the altitude of many points are given, and 

 contour hues are also marked. This excellent system being in operation, it 

 may be asked why this Committee have done anything in the matter ; the 

 reply is very simple and, we think, conclusive. We have not attempted in 

 any way to supplant or encroach on the functions of the Ordnance Depart- 

 ment ; but we have called the attention of our observers to the benefit de- 

 rivable from levelling to the nearest marks, sometimes on their own lodge- 

 gates, yet unknown to them. In this way we have endeavoured to popularize 

 and extend the benefits conferred by these government levels. But there are 

 many places ten, fifteen, or more miles from the nearest benchmark, and for 

 the benefit of observers thus situate, the following arrangements were made, 

 Notice was sent to about 800 rainfall observers that they would be doing 

 good service by taking careful readings of their barometers thrice daily (9 a.m., 

 3 P.M., and 9 p.m.) for ten days. May 24th to June 2nd, 1867, entering them 

 on a printed form sent with the notice, and when filled returning the same to 

 Mr. Symons. These documents were then sorted into two groups, the one 

 comprising only returns from stations at which the index-error of the baro- 

 meter and its height above the mean level of the sea were known, and the 

 other group comprising those stations of which the height was unknown. 

 The returns having been carefully cleared of all errors, intercomparison of the 

 ultimate results has given us a secondary series of altitudes probably correct 

 to 10 or 20 feet, being accurate enough for, all rainfall purposes, these being 

 also available as primary stations should a repetition of the process be 

 necessary at any future time. 



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