406 PHILOSOPHICAL TRANSACTIONS. [aNNO 1788. 



above high-water mark, 14 from the ground, and about 100 yards from the sea. 

 Hence it appears that every night, one only excepted, during that time, the air 

 was coldest at Chislehurst ; and that the mean heat at the sea-shore was equal to 

 that on the tower at Canterbury. In the month of June the cold was still greater 

 in the night at Chislehurst than at any of the other places, excepting where there 

 appeared 2 currents of wind, the upper current from the s. w. and the lower from 

 the N. E. ; at which time also there was the greatest difference between the ther- 

 mometer in the garden and that on the tower. 



The following experiments relate to the variation of local heat in the earth it- 

 self; the diversity of which appears from the different heat of the water issuing 

 from it at different places. It has been conjectured, that the diversity of the 

 temperature of springs may probably depend on their different elevations in the 

 earth, with respect to the level of the sea. Two remarkably deep wells, both 

 near the sea-shore, and not far distant from Canterbury, gave a favourable op- 

 portunity of making experimental inquiry into this matter ; especially as the 

 situation of the 1 springs differed considerably from each other in respect to the 

 level of the sea. One of these is a well in Dover Castle, which is sunk 300 feet 

 through the high cliff of chalk on which the castle stands, and the depth of the 

 well is nearly equal to the height of the cliff from the sea. The other is King's- 

 Well at Sheerness, which was sunk 330 feet through almost one entire stratum 

 of firm clay, where the surface of the ground is only 4 feet above high water. 

 Supposing therefore the spring in Dover well to lie level with the sea, the spring 

 of the well at Sheerness lies 326 feet below it ; a circumstance extremely favoura- 

 ble to the experiment. The temperature of the springs he took in the following 

 manner. After fathoming each well with a line and plummet, he let one ther- 

 mometer down to the bottom, and fixed another on the line, so as to reach to 

 half the depth only, keeping a 3d to take the temperature of the air at the top. 



Sept. 28, 1784. Temperature of the 

 water in the new well in Dover 

 Castle. 

 By the thermometer at the top. . 56^ 



By ditto at the middle . 52 



By ditto at the bottom 484- 



Found the well 360 feet deep with 

 2 1 feet water. 



Oct. 6, 1784. Temperature of the 

 water in King's Well at Sheer- 

 ness. 



By thermometer at the top 53° 



By ditto at the middle 51 



By ditto at the bottom 56 



Found the well 280 feet deep * with 

 180 feet water. 



About noon was the time of day when Mr. S. made the experiments at both 

 places, and the top of the respective wells varying from each other depended 

 wholly on the accidental temperature of the atmosphere at the time; but that 

 the thermometer at half the depth of the well at Dover gave nearly the mean 



* The sand brought up from the bottom of the well, hy the force of the spring, has reduced it to 

 its present depth. — Orig. 



