ON THE BATE OF INCREASE OF UNDERGROUND TEJU'ERATURE. 85 



The well is 8 feet in diameter, with brickwork to the depth of 540 feet, 



and this part of it is traversed by an iron tube 8 inches in diameter, which 



is continued to the depth of more than 1,300 feet from the surface. The 



tube is choked with mud to the depth of about 1,080 feet, so that the 



deepest observations were taken under 20 feet of mud. The temperature 



at 1,100 feet was 69°"9, and by combining this with the surface temperature 



of 49°'9 observed at the Botanic Gardens, Regent's Park, we obtain a rate 



of 1° in 55 feet. These data would give at 250 feet a calculated tem- 



peratui'e of 54'5, whereas the temperature actually observed at this depth 



was 56'1, or 1°"6 higher; the temperature at 300 feet and at 350 feet being 



also 56'1. This seems to indicate convection, but it can be accounted for 



by convection in the 8-foot well which surrounds the tube, and does not 



imply convection currents within the tube. Convection currents are 



much more easily formed in water columns of large diameter than in small 



ones, and the 20 feet of mud at the bottom give some security against 



convection at the deepest point of observation. It is important to remark 



that the increase from 1,050 to 1,100 feet is rather less than the average, 



instead of being decidedly greater, as it would be if there were convection 



above, but not in, the mud. The rate of 1° in 55 feet may therefore be 



adopted as correct. 



Mr. Symons also made numerous observations in this well (VI., TX., 

 XII.) to determine whether the temperature at the depth of 1,000 feet 

 remained exactly the same at all seasons and from year to year. The 

 result was that the changes, if any, were so small as not to be distinguish- 

 able from the necessaiy errors of observation. The research was finally 

 abandoned, because the gradual silting up of the well, which was found 

 to be going on, would itself be competent to account for any small secular 

 change that might have been detected by further observation. 



The strata, consisting of tertiary sti-ata, chalk (586 feet thick), upper 

 greensand and gault, are given in detail on the last page of the third 

 report. 



The Kentish Town temperature at the depth of 400 feet (58°) is con- 

 firmed by observations in Mr. Sich's well at Chiswick (VIII. 159), which 

 is 395 feet deep, and has a temperature varying from 58° to 57°'5. 



The BoOTLE well, belonging to the Liverpool Waterworks, is 1,302 feet 

 deep, and the observations were taken in it during the sinking (XI., XII.) 

 The diameter of the bore is 24 inches, and convection might have been 

 suspected but for the circumstance that there was a gradual upward flow 

 of water from the bottom, which escaped from the upper part of the well 

 by percolation to an underground reservoir near at hand. This would 

 check the tendency to downflow of colder water from the top ; and as the 

 observations of temperature were always made at the bottom, they would 

 thus be protected against convective disturbance. 



The temperature at 226 feet was 52°, at 750 feet 56°, at 1,302 feet 

 59°, giving hj comparison of the first and last of these a mean rate of 1° 

 in 154 feet. The circumstance that the boring ceased for six weeks at 

 the depth of 1,004 feet, and the temperature fell during this interval from 

 58°'l, to 57°"0, would seem to indicate an elevation of 1° due to the heat 

 generated by the boring tool. An assumed surface temperature of 49° 

 (only 0°-9 lower than that of the Botanic Gardens in London), would give 

 by comparison with 57°, at 1,004 feet, a rate of 1° in 125^ feet, and by 

 comparison with 59°, at 1,302 feet, a rate of 1° in 130 fe'et, which last 

 may be adopted as the best determination. The rock consists of the 



