14 REPORT 1875. 



Seventh Report of the Committee, consisting o/Sir W. Thomson, F.R.S., 

 Professor Everett, Sir Charles Lyell, Bart., F.R.S., Professor 

 J. Clerk Maxwell, F.R.S., G. J. Symons, F.M.S., Professor 

 Ramsay, F.R.S., Professor Geikie, F.R.S., James Glaishek, 

 FR.S., Rev. Dr. Graham, G. Maw, F.G.S., W. Pengelly, 

 F.R.S., S. J. Mackie, F.G.S., Professor Hull, F.R.S., Professor 

 Ansted, F.R.S., and Professor Prestwich, F.R.S., appointed fo)' 

 the purpose of investigating the Rate of Increase of Underground 

 Temperature downwards in various localities of Dry Land and 

 under IVater^-. By Professor Everett, D.C.L., Secretary. 



A FEW weeks after the reading of last year's Eeport, another set of observa- 

 tions was received from ilcssrs. Mauget and Lippmann, the engineers of the 

 great artesian well now sinking at La Chapelle, Paris. The water had been 

 undisturbed for a year, this time having been occupied in preparations for 

 tubing the well through its entire depth. 



The exceptionally rapid increase of temperature in the lower part of the 

 well, as indicated in the previous observations, had given reason to suspect 

 that the heat generated by the action of the boring-tool was an important 

 disturbing element. It is now manifest that this suspicion was correct ; for 

 the bottom temperature (660 metres deep), which was 83°-25 Fahr. in the 

 observations of June 1S62, is only 76° Fahr. in the observations of October 

 1863, or 7j° colder than before. At the depth of 600 metres the tem- 

 perature was 75°- 8 and 7o°-4 in the two observations of June 1862, and 

 75° in the observation of October 1863, or about half a degree colder than 

 before. 



At the depths of 500 metres and 400 metres there was no change ; and 

 at the depths of 300 metres, 200 metres, and 100 metres there was an 

 increase amounting to 0°-5 at 300 metres, 0°-8 at 200 metres, and l°-5 at 

 100 metres. 



In explanation of the increase at these smaller depths, Messrs. Mauget and 

 Lippmann remark: — "When last year's observations were made the well 

 had been tubed to the depth of 139-15 metres, but had not been cemented. 

 Consequently the springs which were met with in the tertiary strata com- 

 municated at the base of the tubes with the water of the well. Cement has 

 this year been poured in between all the tubes some days before taking the 

 temperature of the water. This operation has excluded the tertiary springs 

 and permitted the water of the well to resume its normal temperature." 



The new temperatui'e 59°-5 at lOO metres, combined with the new 

 temperature 70° at 660 metres, gives 1° Fahr. for 34 metres, or for 111 feet. 



The old temperature, 58 at 100 metres, combined with the new temperature 

 76° at 060 metres, gives 1° Fahr. for 31 metres, or for 102 feet. 



The temperature 53°-l in the caves of the Paris Observatory, at tlic 

 depth of 28 metres, combined with the temperature 76° at 660 metres, gives 

 1° Fahr. in 27-6 metres, or in 905 feet. 



AU these results differ largely from previous determinations of the rate of 

 increase in the neighbourhood of Paris, which were very harmonious among 

 themselves, and gave a rate of 1° Fahr. in 56 feet (see 1871 Eeport). 



The only source of error that appears possible in the La Chapelle observa- 

 tions is convection by vertical currents in the well. Such action is certainly 



* Head at the Belfast Meeting, 1874. 



