On Underground Temperatures, 



7 



alterations will account for occasional differences in the thermometric 

 gradients, between the observations as originally recorded and as 

 given in these lists, which in most cases will be found referable to my 

 taking a different mean annual temperature to that used by the 

 original observer. 



Height of the Surface. — As this influences both the surface and 

 underground temperatures, it is important that it should be deter- 

 mined with precision. This is not always possible, although the later 

 publications of the Ordnance Survey have in many cases furnished us 

 with levels, which, if not at the precise spot, are sufficiently near to 

 form, with a knowledge of the country, a near estimate; but not 

 unfrequently mines are in out-of-the-way places, where only roughly 

 approximate estimates can be made. Several of the surface heights 

 given in this table necessarily come into this category, and are there- 

 fore subject to future correction; while there are many cases in which 

 I am unable to give even an approximate height. 



Mean Annual Temperature. — This is a point on which it is indis- 

 pensable to have exact information, for it is one in which the differ- 

 ence even of one or two degrees materially affects the calculations. 

 There are yet, however, many of the places in the lists, where 

 satisfactory information is wanting, and the accepted temperature 

 may therefore require revision. For places on the Continent, the 

 works of Arago,* which embody all that was then known of the 

 mean temperature of places, both in France and other parts of the 

 world, are of great value. The publications of the Meteorological 

 Society of Scotland have done much to give us exact data respecting 

 the mean temperature of places in Great Britain, f and I am further 

 indebted to Mr. Robert H. Scott, of the London Meteorological Office, 

 for valuable information respecting the latest recorded mean tempera- 

 ture of places both in this country and on the Continent. There are, 

 besides these, the older and more general tables of Dove. 



Where the temperature of the locality cannot be had, that of some 

 place near and on or near the same level has been taken. Where 

 there is an important difference of height between the place at which 

 the surface temperature has been taken and the site of the under- 

 ground experiment, an approximate allowance of one degree F. 

 (+ or — ) is made for every 300 feet of difference of level. When, 

 however, as is sometimes the case, a mean annual surface temperature 

 is not obtainable, a near approximation may often be made by means 

 of ordinary surface wells of about 40 to 50 feet deep, and springs. , 

 This plan has been, especially in the earlier observations, frequently 

 adopted. 



What is really desirable is that the datum line whence to commence 



* " Notices Scientifiques," vol. v, p. 518. Edit. 1857. 

 f Especially the number of Journal for November, 1883. 



