MEAN TEMPERATURE. 123 



bj simple reference to the position of the sun, and 

 in general can only be ascertained with certainty 

 by actual observation. 



Suppose that, at any given place, the total 

 amount of heat received in a year was equally 

 distributed over equal intervals of time, so that, 

 for instance, in one hour exactly as much heat 

 Was taken in as during another hour, then the 

 number thus obtained would be the expression 

 of the Mean Temperature of the Ground at that 

 place. This may be determined approximately by 

 placing the end of a correct thermometer an inch 

 deep in the ground, so that the bulb at least 

 should be covered with earth, in a dry, shaded 

 spot. Xow let the position of the mercury be 

 noted after equal successive intervals of time — for 

 instance, hour by hour — and take the arithmetic 

 mean of all the observations. This mean-value 

 deduced from the observations of a day, of a 

 month, of a season, and lastly of a whole year, 

 gives the mean temperature of the day, the month, 

 the season, the year. You will find that the mean 

 yearly temperature of a place is subject, from year 

 to year, to but very slight variations, which more- 

 over in the course of years will perfectly compen- 

 sate each other. The total annual income of heat 

 is therefore equal to the outgoings. Were this not 

 the case, the annual mean must either increase or 

 diminish in the lapse of years. 



