38 



Prof. J. Prestwich. 



gives a difference of 31° for the whole depth, or 64J feet increase 

 for each degree F. If it stood by itself this might appear nothing 

 remarkable, yet it is evident, from the series of observations that were 

 taken in this bore-hole, that the temperature or rate of increase at the 

 depth of 100 feet, which was equal to 1° in 10 feet, and at 300 feet to 

 1° in 28 feet, is excessive, and that this excess can only have been 

 acquired through loss of heat by convection currents at the bottom and 

 a corresponding gain at the top, and thus making the readings too high 

 at top and too low at bottom. If, however, we take an intermediate 

 station at a depth of 1000 feet, where the temperature averages 69*2°, 

 we get a mean, and more probably truer, rate of increase with depth 

 of 47 feet per degree. 



Owing to the large diameter of the bore, to the many water-levels 

 in the rocks, and to the circumstance that when the observations 

 were made there was no sufficiently free outflow of water on the surface, 

 such as would check the convection currents, there can, I think, be no 

 doubt that the temperature of the well at Bootle ("No. 153), sunk in 

 the New Red Sandstone at Liverpool, is influenced both by convection 

 currents and by the influx of waters at different levels. 



It is clear, therefore, that great uncertainty attaches to all observa- 

 tions made in bore-holes with standing water, the error being in 

 proportion to the diameter of the bore-hole ; and that where experi- 

 ments have been made without plugging, all the deep temperature 

 readings will be too low. Even with this precaution, it may be a 

 question whether the bottom water and that of the adjacent rock may 

 not have had their temperature permanently lowered before trial. 

 This might be remedied to some extent by the use of more than one 

 plug, and the stoppage of the circulation throughout the whole depth 

 of the bore-hole for a certain time. 



There are, however, some artesian bore-holes, where the sources of 

 error have been reduced to a minimum. Amongst those are — 



1. Kentish Toivn (No. 129). — These careful experiments were carried 

 on for some years by Mr. Symons. From the circumstance that the 

 mud at the bottom of the bore-hole into which the thermometer was 

 sunk, was free from convection currents, the results obtained show, 

 in all probability, a near approximation to the normal temperature. 

 At the same time the long period that the well had stood neglected 

 allowed the play of those currents in the water standing in the tube 

 above the mud, and this may possibly have effected a slight reduction 

 of temperature, but it cannot be very material. 



2. Richmond. — The observations here were made by Professor Judd 

 with standard instruments. The overflow was too small to give the 

 correct temperature, though enough to check convection currents, 

 and consequently the temperature was ascertained by letting down a 

 thermometer to the bottom of the well. 



