36 



Prof. J. Prestwich. 



strata, where the rocks are, as a rule, softer, more permeable to 

 water, and less disturbed. 



Many of the interfering causes difficult to eliminate in the case of the 

 Mines, do not exist with the Artesian Wells. The causes of interfer- 

 ence in the latter are reduced mainly to two, namely, pressure on the 

 instruments and convection currents. The early experiments, where 

 no precautions were taken against pressure, are consequently un- 

 reliable. Walferdin introduced improved and protected instruments, 

 although in some previous cases, as in Marcet and De la Rive's 

 observations, protected thermometers had been used. The need of 

 protection against convection currents had also not escaped attention, 

 but it was not until the later observations, instituted by the Com- 

 mittee of the British Association, were made, that more efficient safe- 

 guards were introduced to protect more completely against the subtle 

 influence of these currents. 



It is clear that we must reject all the early experiments made with 

 unprotected thermometers ; and it is not certain whether also a large 

 number of those made with protected thermometers, but without pro- 

 tection, and sufficient protection, against convection currents, should 

 not also be rejected. 



In large bore-holes the disturbance from this cause is so great 

 that the consequences are at once sufficiently apparent to oblige 

 the rejection of the observations. The ordinary deep artesian 

 wells of Paris, such as Grenelle and others, all agree in showing 

 a rate of increase of 55 to 58 feet per degree F. ; but the 

 great bore-hole (4J feet in diameter) through the same strata in 

 another part of Paris (La Chapelle St. Denis), where the water does 

 not overflow, gave a rate of increase of 39-J feet per degree for the first 

 100 metres (328 feet), which is too rapid, whilst at the depth of 

 660 metres (2165 feet) the rate of increase was only 1° in 84 feet. 

 This is clearly due, as stated by Professor Everett, to the over- 

 heating of the upper part of the column of water and to the cooling of 

 the lower part by the action of convection currents. In the report of 

 a deep boring at Moscow it is stated, but without explanation, that 

 from 350 feet to the bottom at 994 feet the temperature was nearly 

 constant at 10' 1° C. (50-2° F.). The diameter of the bore-hole is not 

 given, but I judge from other circumstances that it is not less than 

 2 feet, and can only account for the uniformity of temperature by the 

 action of convection currents. The mean annual temperature of 

 Moscow is 39 - 4°. 



Professor Everett* also directs attention to the manifest action of 

 convection currents in a shaft at Allendale, about 350 feet deep, with 

 nearly 150 feet of water, in which the temperature was practically the 

 same at all depths. 



* Brit. Assoc. Keports for 1871 and 1869. 



