If we are to compare these rates of temperature change, we must have some definite scale of measurement. Thus, for example, in speaking of the wind velocity we say the velocity of the wind is so many miles per hour; in describing the grade of a railroad we say it is so many feet in a mile. In dealing with these temperature changes, we adopt a similar scheme. We say: The rate of temperature decrease is so many degrees Fahrenheit in a distance of one latitude degree (about 70 miles). In order to make our measurements, we use a scale of _latitude degrees_, just as, in calculating railroad grades, we must have a way to measure the miles of track in which the ascent or descent of the roadbed is so many feet. Take a strip of paper 6 inches long, with a straight edge, and lay this edge north and south at the middle of the weather map, along a longitudinal or meridian line. Mark off on the strip of paper the points where any two latitude lines cross the meridian line. These latitude lines are five (latitude) degrees apart. Therefore divide the space between them on your paper into five divisions, and each of these will measure just one latitude degree. Continue making divisions of the same size until you have ten altogether on the strip of paper. Select, on any weather map, some station lying between two isotherms at which you wish to measure the rate of temperature decrease. Take, for instance, Buffalo, N. Y., on the first day. What you want to find is this: What is the _rate of temperature decrease_, or the _temperature gradient_, at Buffalo? Lay your paper scale of latitude degrees through Buffalo, from the isotherm of 10° to the isotherm of 0°, and as nearly as possible at right angles to the isotherms.[3] Count the number of latitude degrees on your scale between the isotherms of 10° and 0°, on a line running through Buffalo. There are, roughly, about two degrees of latitude in this distance. That is, in the district in which Buffalo lies, the temperature is changing _at the rate_ of 10° Fahrenheit (between isotherms 10° and 0°) in two latitude degrees. As our standard of measurement is the amount of change of temperature in one latitude degree, we divide the 10 (the number of degrees of temperature) by the 2 (the number of degrees of latitude), which gives us 5 as the rate of decrease of temperature per latitude degree at Buffalo, N. Y., at 7 A.M., on the first day of the series. The temperature gradient at Buffalo is therefore 5. The rule may be stated as follows: Select the station for which you wish to know the rate of temperature decrease or temperature gradient. Lay a scale of latitude degrees through the station, and as nearly as possible at right angles to the adjacent isotherms. If the station is exactly on an isotherm then measure the distance _from_ the station to the nearest isotherm indicating a temperature 10° lower. The scale must, however, be laid perpendicularly to the isotherm, as before.