152 THE WORLD. 



tilde. The true distribution of heat over the earth's surface can 

 therefore only be determined by a long series of observations. 

 Baron Hurabolt with unwearied zeal, has collected the data for, 

 and laid the foundation cf, a scientific meteorology. The instru- 

 ment employed to measure the intensity of heat, called a ther- 

 mometer is to well known to need any description here. The 

 thermometer in ordinary use is what is called Fahrenheit's, the 

 scale being graduated to show 212 for the heat of boiling wa- 

 ter, and 32 for the temperature of melting ice, or freezing water. 

 The zero or commencement of the scale, is the temperature of a 

 mixture of salt and ice, or snow, and which was once supposed to 

 be the greatest artificial cold. The thermometer called Reau- 

 mer's is used in some parts of the continent of Europe, the freez- 

 ing point of water being zero, or the commencement of the scale, 

 and the space between this and the boiling point of water is di- 

 vided into 80. The thermometer now used in France, and the 

 greater part of the continent of Europe, is called Centrigrade ; 

 the scale of this thermometer is graduated into 100 degrees from 

 the freezing, to the boiLng point of water ; this division of the 

 .scale appears the most natural, and has been adopted by law in 

 the state of New York. 



In employing the thermometer to observe the general tempera- 

 ture of the air at any particular season of the year, it will gener- 

 ally be sufficient to make two observations in the morning, viz: 

 at 4h, and lOh, and two in the afternoon at the same hours, the 

 mean of the observations will give the mean temperature for 

 the day very exactly ; thus, suppose the observations made at 

 these hours to be 50, 80, 90, and 60, adding these all together, 

 and dividing their sum 280, by 4 gives 70 for the mean tem- 

 perature of the day. When we know the mean temperature of 

 all the days of a month, we can in like manner determine the 

 mean temperature of that month. We can likewise determine 

 in a similar manner the mean temperature of the year, or of 

 summer, and winter. The mean annual temperature, of a place 

 not subject to very great local changes, such as the clearing up 

 of forests, or drying up of streams and rivers, is very nearly con- 

 stant. Thus, the extreme difference of mean annual tempera- 



