1851.] HOPKINS ON CHANGES OF CLIMATE. 57 



depth, it will only indicate a variation of temperature proceeding 

 from day to day as the seasons advance ; but if the mean of all the 

 observations on each of these thermometers be taken for the whole 

 year, it will be sensibly the same for both, and will give the earth's 

 mean annual superficial temperature. If again a third thermometer 

 be placed a few inches above the ground, it will indicate the variable 

 temperature of the atmosphere, and the mean of all the observations 

 on this instrument for the whole year will give the mean annual tem- 

 perature of the atmosphere. This mean again will be sensibly the 

 same as the mean results obtained from the two other thermometers, 

 notwithstanding that the laws of variation for the three instruments 

 respectively are so very diiferent. Consequently, the earth's mean 

 annual superficial temperature is equal to the mean annual tempe- 

 rature of the atmosphere close to the earth's surface. 



Again, if we place a fourth thermometer at a depth greater than 

 about 2 feet, and not exceeding, in this latitude, 60 or 70 feet, 

 it will indicate (like the second of the above-mentioned thermo- 

 meters) a temperature changing only with the advancing seasons, and 

 (except for depths not much exceeding 2 or 3 feet) with great regu- 

 larity. If, instead of a single instrument, we should place a series 

 of thermometers at different depths from 2 feet to about 70, their 

 indications would be very different, although all would follow in 

 a similar manner the changing temperature of the seasons. Each 

 would have its maximum and minimum heights during the year, but 

 the times at which these maxima and minima would occur would be 

 different for each instrument. They would be foinid earliest in the 

 higher instruments (soon after the hottest part of the summer, and 

 the coldest part of the winter respectively), and later in the lower 

 instruments according to their depths. The extent of the oscil- 

 lations from the highest to the lowest temperatures would also be 

 different for each instrument. It would be greatest for the upper 

 ones and would decrease according to the depth in the lower ones, 

 until at the depth of about 60 or 70 feet the oscillations would 

 be no longer sensible, the instrument always indicating the same in- 

 variable temperature. 



The mean annual temperature indicated by every one of the ther- 

 mometers above-mentioned at depths less or greater than 2 feet 

 would be nearly the same. Independently of accidental circumstances 

 which might affect those immediately beneath the surface of the 

 ground, these mean temperatures would be rather greater for the 

 lower than for the upper instruments in proportion to their depths, 

 but that given by the lowest one (which would be its stationary 

 temperature) would not exceed the mean temperature given by the 

 upper one, by more than about 1° Fahr. 



Finally, let us suppose our series of thermometers continued down- 

 wards along the same vertical line, at depths greater than 60 or 

 70 feet. Each instrument would indicate an unchanging temperature 

 during the whole year, but the greater the depth of the instrument 

 below the earth's surface, the higher would be the temperature in- 

 dicated by it, the difference between the indications of any two 



