Is the pressure constant (_i.e._, are the readings always the same) or does it vary? If it varies, is there any apparent system in the variations? Is there a tendency to a daily maximum? To a daily minimum? If so, about what time do these occur, respectively? What is the average variation (in inches and hundredths) in the course of a day? What is the greatest difference in pressure which you have observed in a day? What is the least? Does the pressure seem to vary more or less in the colder months than in the warmer? Has the height of the mercury column any relation to the weather? Are we likely to have rainy weather with rising barometer? Is the velocity of the wind related to the pressure in any way? How? Can you make any general rules for weather prediction based on the action of the barometer? What rules? =Tabulation of Observations.=——The tables suggested in the preceding chapter can be used unchanged with the simple instruments just described. =Summary of Observations.=——At the end of each month summarize your instrumental observations in the following way:—— _Temperature._——Add together all your temperature readings; divide their sum by the total number of observations of temperature, and the quotient will give you a sufficiently accurate _mean_ or _average_ temperature for the month in question. It is to be noted that the mean monthly temperatures obtained from these observations will be much more accurate if the thermometer readings are made at 7 A.M. and 7 P.M., at 8 A.M. and 8 P.M., etc., and the mean of these is taken; or if the mean is derived from the maximum and the minimum temperatures, discussed in Chapter III. This _mean_ temperature should be written at the bottom of the temperature column, and marked “Mean.” The mean monthly temperature is one of the important meteorological data in considering the climatic conditions of any place. _Wind._——Determine the frequency of the different wind directions by counting the total number of times the wind has blown from N., NE., E., etc., during the month. The wind which you have observed the greatest number of times is the _prevailing_ wind. It may, of course, happen that two or three directions have been observed an equal number of times. The number of calms should also be recorded. _Rainfall._——The total monthly precipitation is obtained by adding together all the separate amounts of rainfall noted in your record book, and expressing the total, in inches and hundredths, at the bottom of the rainfall column. You now have the means for comparing one month’s rainfall with that of another month, and of seeing how these amounts vary.