OF ARTS AND SCIENCES. 347 



abscissas. In practice, a simple method of proceeding is adopted : 

 lines are drawn on the storm stratum (tracing-paper) to represent the 

 path of every station through the storm, and the name of the station 

 is written at one end of its line : then a time scale is prepared to 

 measure the abscissas, its unit being the average distance traversed by 

 the storm in an hour ; for a storm moving from west to east, as is the 

 general rule, the scale is numbered from right to left; it is next laid 

 on a certain station line, parallel to the middle path, and placed stt 

 that the time of rain beginning for this station falls on the rain-front 

 line ; then all the observations from that station can be marked down 

 opposite their proper hour and minute on the time scale, and they will 

 thus fall in their proper place in the storm. After plotting a good 

 number of reports in this way, the composite portrait shows that the 

 area an hour or so in front of the rain is occupied with records show- 

 ing high temperatures, clear sky with clouds from which thunder is 

 heard rising in the west ; gentle winds, as a rule from a southerly 

 quarter ; nearer the storm the clouds cover more of the sky, and the 

 temperature falls a little ; close in front of the rain the wind-squall 

 appears, blowing out from the storm ; as the rain is reached, the wind 

 dies away, the temperature falls rapidly, the thunder grows louder, and 

 lightning strokes appear ; half an hour or more, with rain still falling 

 gently, the western clouds break up, and blue sky appears ; the tem- 

 perature rises slowly, and the thunder dies away as the storm moves 

 off; and rainbows appear on its back as the sun shines out. It soon 

 becomes evident that there is really a systematic distribution of certain 

 elements of the storm that are susceptible of legitimate averaging ; and 

 from their graphic representation in the composite portrait, the proper 

 areas and intervals for averaging may be chosen. In this way, the 

 fullest use may be made of the most varied records. 



