46 TRANSACTIONS OF THE ILLINOIS 



winds blowing from them. This is not strictly correct. Such winds 

 move in winding spirals. Those flowing from an area of high pressure, 

 can be represented by a spiral receding from the center or axis ; and 

 those toward an area of low p;essure, by a contracting spiral. 



Draw a double spiral resembling the letter S reversed, and you will 

 have a graphic illustration of such winds in the northern hemisphere, 

 while a spiral drawn so as to resemble the letter S will illustrate such 

 winds in the southern hemisphere. Re])resent the direction of the wmd 

 by arrows pointing in opposite directions along the line of the first dia- 

 gram, — that is, the one representing the winds in the northern hemi- 

 sphere, and you will find the direction of the out-bowing wind is a direct 

 spiral, that is from left to right or with the hands of a watch with its face 

 lying upwards ; while the in-bhiuing wind describes a retrograde spiral, 

 that is, from right to left, or contrary to the hands of a watch. If sim- 

 ilar arrows are placed alongside of the other double spiral to represent 

 the direction of the wind in the southern hemisphere, it will be seen 

 that the direction of corresponding winds is reversed. 



The practical point now is to ascertain in what direction the areas 

 ,, of high and low pressure lie. In whatever direction the winds may be 

 blowing, turn your back square to it ; then to the left will be the area of 

 low pressure, and to your right the area of high pressure. For instance, 

 if it be a south wind, then you will face the north ; your left hand will 

 be west and your right hand east. Consequently, the area of low pres- 

 sure will be west, and that of high east. But an electric wave has 

 longitudinal extension, while ii; may have only one point that is a storm- 

 center. If therefore the wind be south, while a wave of low pressure is 

 approaching from the west, then the storm-center or nucleus of greatest 

 disturbance will lie about half-way between the north, — the way you 

 are facing, — and the west, that is in the northwest. If in such case the 

 storm-center only covers a small area, there will be no rain ; since all 

 hydrometeors in the northwestern part of the Mississippi valley, move 

 on a line nearly due east from the west. 



Suppose that the wind is southeast, then the point of the compass 

 toward which your face will be turned is northwest, and your left shoul- 

 der will be turned to the southwest. In that case, because the wind 

 describes a retrograde or left hand spiral, the storm-center will lie in the 

 west ; and the probability is, you will be in its path eastward. Your 

 safest guide now will be the indications of your barometer. If the wind 

 remains steady in the southeast, and your barometer keeps falling, you 

 are on the line of the storm-center. If the wind shifts to the east, the 

 center is passing south of you ; if to the south, then it is passing north 

 of you. When the storm-center passes south, the wind shifts by way of 

 the east and north to northwest; if it pass north, then the wind shifts 

 by way of the south and southwest to the west. 



If the storm-center approaches you, the wind will be steady in the 

 southeast until you are in the center, when there will be a lull for a few 



