78 INTRODUCTION. 



general data on the mean warmth of the earth, 

 than the most ingenious theoretical speculations, 

 and the most laborious and tedious thermometrical 

 observations on shore, where the mean temper- 

 ature is clianged by so many contingent causes.* 

 The temperature of the water at the surface is 

 easily found, by a common thermometer let down 

 a few feet, and immediately drawn up again. To 

 discover the temperature far below the surface, 

 many methods have been proposed ; the chief fea- 

 ture in most of which is to use the thermometer 

 with large balls, which are but slowly affected by 

 a change of temperature, and which may be sur- 

 rounded with wooden caps, and other substances 

 that are bad conductors of heat j and also to leave 

 the instrument a long time below the surface. But 

 the thermometographs are much more convenient 

 for these experiments, especially that which is 

 called from its iventor. Six-thermometer, and in- 

 dicates the maximum of a preceding change of 

 the temperature. It is to be wished, that a navi- 

 gator who is provided with such an instrument, 

 would not suffer any calm to pass over, without 

 letting down this instrument to various great 

 depths. Particular pains should be taken to find 

 out that depth at which, as preceding observ- 



* We except experiments on the temperature of springs 

 on land, which, when opportunity offers, are never to be 

 neglected, for the purpose of determining the temperature of 

 the earth. 



