The instructions in the text for measuring rates of pressure-decrease are, that these rates shall be recorded as so many hundredths of an
inch of change of pressure in one latitude-degree. This is done for the sake of simplicity. If this rate is expressed in hundredths of an inch of pressure in a quarter of a latitude-degree of distance, the numerical value is the same as if expressed in millimeters of pressure per latitude-degree of distance.
CHAPTER VIII.
The fact of the prevalence of different kinds of weather over the country at the same time is of great importance. It should be strongly emphasized by the teacher in the course of the discussion of the maps of weather distribution. Additional exercises of the same sort may be given to advantage, by letting the class plot and study the weather signs taken from any current weather map. Instructive lessons may be taught by talking over, in the class, the different ways in which people all over the country are affected by the character of the weather that happens to prevail where they are.
CHAPTERS IX-XVIII.
The correlation exercises will, as a whole, teach few entirely new facts to the brighter scholars who have faithfully completed the preceding work in observations and in the construction and study of the daily weather maps. These exercises do, however, lead to detailed examination and to the careful working out of the relations which may have been previously noticed in a general way only. They give the repeated illustration which is necessary in order to impress firmly on the mind the lesson that the weather map has to teach.
It is a good plan to let different scholars work out the problems for different months. The results reached in each case should be discussed in the class, and thus each member may have the double advantage of working out his own problem, and of profiting by the work done by his fellows. Throughout these exercises care should be taken to have weather maps of all months studied. The exercise on the correlation of the velocity of the wind with the pressure
cannot be undertaken unless the work on temperature and pressure gradients (Chapters V and VII) has been completed.
CHAPTERS XX-XXV.
It is not expected that any one scholar can accomplish all that is here outlined. Examples may be selected from the list, as opportunity offers, so that each scholar shall become familiar with several problems.