ON UNDERGROUND TEMPERATURE. 



185 



" It is well knoTvn that in the solid crust of the earth the influence of sea- 

 sons penetrates but a slight depth, say 60 feet ; but it occurred to me that 

 this might not hold good in the case of such an opening as the Kentish Town 

 well. I therefore decided on commencing my observations at midwinter, 

 contiuuiug them regularly to midsummer, and then repeating every obser- 

 vation ; those at each depth will therefore have been taken twice at exactly 

 opposite seasons, and at intervals of six months. The necessity for this 

 extreme care did not appear obvious at first, and it seemed as if the various 

 precautions against the ingress of atmospheric temperatures had rendered it 

 superfluous ; but during recent hot periods its desirability has become abun- 

 dantly manifest : the temperature at a depth of 50 feet was 49°-2 in Janu.ary 

 and 54°-l in July ; that at 100 feet was 51° in January and 54°-3 in July ; 

 at 150 feet 52°-! in January and 54°-7 in July. It is therefore evident that 

 under the cii'cumstances existing at Kentish Town, it is more easy to deter- 

 mine accurately the temperature at great depths than at the lesser ones. It 

 is certain that but for the precautions taken, and the unusual mildness of the 

 winter, the temperature at 50 feet would have been much below 49°-2. 

 Whence it further appears that though a single observation at depths below 

 200 feet will probably give accurately the true temperature at any selected 

 depth, yet in shafts and bores similarly circumstanced to that now under 

 notice, very discordant residts may be obtained at lesser depths. Moreover, 

 it is obviously impossible, bj' anj' but long-continued observations, to deter- 

 mine accurately the surface-temperature of the ground, or the equivalent of 

 a depth of feet ; it may therefore be expedient, for the purpose of com- 

 pleting the series, to assume that the mean temperature of 

 the surface of the soil at Kentish Town, 187 feet above mean 

 sea-level, is identical with that of the air at Green'ndch (49°) 

 at 159 feet above the sea, and it is satisfactory to find that 

 the observations hitherto made agree perfectly with this 

 hypothesis. Although, as we have already stated, the ex- 

 periments are by no means concluded, it may be convenient 

 to tabulate the results hitherto obtained. Being impressed 

 with the high importance of accurate knowledge of the 

 rate and amount of seasonal change in the shaft, Mr. Symons 

 designed, and Mr. Casella (aided in part by Messrs. Silver & 

 Co.) constructed, a very delicate thermometer, which was 

 cased 5 inches thick in felt and non-conducting materials, 

 and enclosed in an ebonite box, as in the annexed section ; 

 the non-conducting powers of this instrument were such that 

 on one occasion it was raised into the observing-room show- 

 ing a temperature of 51°-14, and after being in a tempera- 

 ture of 60° for thirty-five minutes it had only risen 0'^-02. 

 By this means it was therefore possible to bring up the exact temperature of 

 any required depth, uninfluenced by the warmer or colder strata through 

 which it might have to pass. It was regularly obseiwed for some time during 

 the present spring, and the following readings obtained : — 



were experimented on. Sir W. Thomson's is that which is designated 'No. 9645. A 

 mercurial maximum thermometer, on Professor Phillips's plan, enclosed in a strong outer 

 tube containing a little spirit of wine, and hermetically sealed.' " 



