_State of the Sky._——In connection with the more advanced records described in this chapter, the observations of cloudiness should record the number of _tenths_ of the sky cloudy, as closely as the amount can be estimated by eye, instead of indicating the state of the sky as _cloudy_, _fair_, etc. A detailed record of cloudiness in tenths gives opportunity to determine the _mean cloudiness_ for each month, by averaging, as in the case of the other means already described. If nephoscope observations are made, the monthly summary may include the _mean direction of cloud movement_ for each month. This is obtained by adding together all the different angular measurements of directions of cloud movement, and dividing by the whole number of such observations. By means of your monthly summaries compare one month with another. Notice how the means and the extremes of the different weather elements are related; how they vary from month to month. Are there any _progressive_ changes in temperature, cloudiness, precipitation, etc., from month to month? What are the changes? Summarize, in a short written statement, the meteorological characteristics of each month as shown by your tables. PART III.——EXERCISES IN THE CONSTRUCTION OF WEATHER MAPS. CHAPTER IV. THE DAILY WEATHER MAP. The first daily weather maps were issued in connection with the Great Exhibition of 1851 in London. The data were collected by the Electric Telegraph Company and transmitted to London over its wires. These maps were published and sold daily (excepting Sundays) from Aug. 8 to Oct. 11, 1851. The first official weather map of the United States Weather Service was prepared in manuscript on Nov. 1, 1870, and on Jan. 14, 1871, the work of manifolding the maps for distribution was begun at Washington. Previous to the publication of this government map, Professor Cleveland Abbe had issued in Cincinnati, with the support of the Chamber of Commerce of that city, the first current weather maps published in the United States (Feb. 24 to Dec. 10, 1870). In France, daily weather maps have been published continuously since Sept. 16, 1863.