Divide the number of degrees of difference of temperature between the isotherms (always 10°) by the distance (in latitude degrees) between the isotherms, and the quotient is the _rate of temperature decrease per latitude degree_. Or, to formulate the operation: _R = T / D_, in which _R_ = rate; _T_ = temperature difference between isotherms (always 10°), and _D_ = distance between isotherms in latitude degrees. Thus, a distance of 10 latitude degrees gives a rate of 1; a distance of 5 gives a rate of 2; a distance of 2 gives a rate of 5; a distance of 4 gives a rate of 2.5, etc. [Footnote 3: Unless the isotherms are exactly parallel, the scale cannot be at right angles to both of them. It should, however, be placed as nearly as possible in that position.] Determine the rates of temperature decrease in the following cases:—— _A._ For a considerable number of stations in different parts of the same map, as for each of the six days of the series. And, using the school file of weather maps, _B._ For one station during a winter month and during a summer month, measuring the rate on each map throughout the month and obtaining an average rate for the month. _C._ For a station on the Pacific Coast, and one on the Atlantic Coast during the same months. _D._ For a station on the Gulf of Mexico, or in Florida, and one in the Northwest during a winter month. _E._ For a station in the central United States, and one on the Pacific Coast, the Gulf Coast, and the Atlantic Coast, respectively, during different months of the winter and summer. The determination of the rates of temperature decrease under these different conditions over the United States prepares us for an appreciation of the larger facts, of a similar kind, to be found on the mean annual and mean monthly isothermal charts of various countries, and also of the whole world.