The form of table given at the end of this chapter is intended merely as a suggestion, and not as a rigid scheme to be adopted in every school. In using the instruments here described, practice with the maximum and minimum thermometers (in addition to the simpler work of Chapter II) may be given before any attempt is made to have the class use the psychrometer. And in using the psychrometer one week may well be given to the determination of the dew-point alone, before the wet and dry-bulb readings are employed to determine the relative humidity. Absolute humidity, which is not referred to in this chapter, may, if the teacher deem it advisable, be added as another weather element for study. A refinement in the notes on the state of the sky is suggested, viz., that cloudiness should be recorded in tenths of the sky covered by clouds. This is an advance over the earlier, less accurate cloud observations, and is in line with such a progressive scheme as has been recommended. This book is not intended to present a rigid scheme of observational work in meteorology, alike for all schools, but rather to make suggestions for the guidance of teachers in laying out such a course as may fit their own cases. Under the heading _Summary of Observations_ only a few of the most important climatic elements have been noted. The list may easily be extended by the addition of such data as the following: For temperature, mean diurnal range; mean diurnal variability (the mean of the differences between the successive daily means). For humidity, monthly mean absolute humidity. For precipitation, the maximum daily precipitation; the number of rainy and snowy days in every month, the number of clear, fair, and cloudy days in every month; the mean frequency of rainfall in every month (number of rainy days divided by the total number of days); the number of days with thunderstorms, etc. It is important that the monthly summaries should be discussed in the class, and that the scholars should give verbal statements as to the numerical results which they have obtained. In this way the work will have a living interest, which the mere compilation of summaries does not possess. CHAPTER IV.