348 SCIENTIFIC RECORD FOR 1884. 



summary, bearing especially upon the Boen or wind-gusts that accom- 

 pany the thunder-storm. 



Throughout its entire course the Boen was associated with the exist- 

 ence of an extraordinary well-developed change in pressure and temper- 

 ature. This temperature distribution, as shown by closely compressed 

 isotherms, corresponded to a very steep barometric gradient which aver- 

 aged 9 millimeters at the place of most compressed isotherms. The 

 object of the ibllowing is to explain how this extreme condition could 

 originate. In the region which was later in the day to be visited by the 

 Boen, there was already on the 8th of August a feeble southerly cur- 

 rent and strong insolation with a temperature high above that pre- 

 vailing to the westward, and which in Leipsic rose to 29.7 C, by noon of 

 the 9th. This high temperature relative to the surroundings must have 

 given rise to the formation of a low pressure at the base of the warm 

 column of air. But that the isotherms should be so closely crowded is 

 due to the fact that on the border of the warm and cold air, rainfall was 

 caused by the uplifting of the former, which materially cooled this 

 region and thereby propagated the phenomena toward the direction of 

 the warm mass of air. With the heavy rain that thus originated air must 

 have been dragged down mechanically with the drops and also muvst 

 have >sunk down in consequence of its lower temperature and greater 

 density. The dynamic warming of this air could onl^- be slight because 

 of its mixture with water, and the downfalling cold air must have exerted 

 a strong uplifting effect upon the warm air below and adjoining it, and 

 thus the phenomena continually increased in severity. Thus, precisely 

 on the limit of the warm region the production of cold was mostintense; 

 in fact we see the greatest crowding of isotherms there where the pre- 

 cipitation is heaviest in the Boen, and where it partly fell in the form 

 of hail. The difference of temperature between Neustadtand Segeberg 

 at 2 p. M. was 26o.l— 140.6 or llo.S C, for only 39 kilometers distance. 

 Koppen attributes the retardation of the lower stronger gradients to 

 friction at the surface of the ground, thereby causing the steepness of 

 the change of pressure. The effect of this steep gradient is felt almost 

 entirely in the acceleration given to the outflowing mass of air, because 

 its movement is nearly in the direction of this gradient. [The gradient 

 represents a constant force pushing outwards like gravity pulling down- 

 wards ] The slight value of the angle between the gradient and the 

 wind is ascribed partly to the acceleration, partly to the circumstance 

 that the air drawn into the Boen already has amotion nearly in the di- 

 rection of the gradient. This distribution of pressure by reason of the 

 great horizontal temperature gradient must change very rapidly with 

 the altitude, and Dr. Koppen computes that, at an altitude from 600 to 

 700 meters, the irregularities in the isobars have disappeared and they 

 have become simple ellipses inclosing the principal depression. At the 

 place where the warm air is lifted up in front of the Boen thick masses 

 of clouds must form, which move with the general air-current towards 



