ON THE ASSESSMENT OF DIRECT TAXATION. 27 



casilj"- be defined. The agreement which they trust eventually to trace 

 between the observed temperatures and the experimentally determined 

 thermal properties of the locally predominating rocks is liable to be masked 

 and concealed by causes of disturbance of so many unknown and unsiispccted 

 kinds, that plain and obvious corroborations are not frequently to be expected ; 

 and the nature of those causes which principally tend to disturb the results 

 Avill probably become better known by the progress of further comparisons 

 such as the Committee is now endeavouring to pursue. While it was thus 

 anticipated by Prof. Everett*, from the slow rate of temperature-variation 

 from the surface observed in the rocky excavations of the Mont-Cenis tunnel, 

 that quartz (which is a principal ingredient of the rock) would prove to have 

 a high thermal conductivity, this property is now also found to belong to 

 rock-salt, through which the Sperenberg boring passes with an average rate 

 of temperature-variation (1° F. in 51 English feet) scarcely differing sensibly 

 from the mean rate obtained from a mass of similar observations taken in 

 other places and recoi'ded by the Underground Temperature Committee. 

 The apparent contradiction presented by these two cases may possibly proceed 

 from a more rapid local rate of variation of temperature in the neighbourhood 

 of Sperenberg than around Mont Cenis ; and the fact that in the first GO 

 fathoms of ordinary strata overlying the rock-salt the observed rate of 

 variation was slower than below (contrary to what would be expected from 

 the relative conductivities of the superincumbent strata and the underlying 

 masses of rock-salt), is said, in Herr Bunker's description of the obser- 

 vations, to be probably accounted for by the intrusion into the boring near 

 its mouth of the Avaste warm water of the engines on the surface. The 

 effect, it may be observed, of a highly conducting mass, like that of the deep 

 bed of rock-salt here penetrated, by diminishing the local resistance and 

 increasing the flow of internal heat outwards through the Sperenberg strata, 

 would be to cause the local rate of variation of temperature in this locality 

 to be abnormally rapid ; and perhaps this may explain why a slow rate of 

 variation is not observed in this instance, from the great depth of the ex- 

 cellently conducting rock-salt formation, which considerably exceeds 3000 feet. 

 The Sperenberg boring thus presents examples of secondary conditions which 

 will perhaps prove to be in good agreement (instead of, as they at first appear 

 to be, somewhat at variance) with the results of the Committee's observations. 



Report of a Committee, consisting o/the Right Hon. J. G. Hubbakd, 

 M.P., Mr. Chadwick, M.P., Mr. Morley, M.P., Dr. Farr, Mr. 

 Hallett, Professor Jevons, Mr. Newmarch, Professor Leone 

 Levi, Mr. He\wood, and Mr. Shaen [with power to add to their 

 number), appointed for the purpose of considering and reporting on 

 the practicability of adopting a Common Measure of Value in the 

 Assessment of Direct Taxation, local and imperial. By Mr. 

 Hallett, Secretary. 



YorR Committee, appointed to inquire into the subject of a Common Measure 

 of Value in Direct Taxation, have proceeded in this inquiry, have considered 

 the matters to them referred, and have agreed to the following Report : — • 



* See these Eeports, vol. for 1875, p. 16, r.ote at bottom of the page. 



