METEOROLOGY. 345 



record of the storms — the blank spaces can be rapidly filled out, snggest- 

 iuff, as they do, to the most inexperienced observer, the items thatBe- 

 zold desires. The reports received during 1883 amount to over 5,000, 

 and come from over 300 stations. These reports are studied by entering 

 ajjon a chart for each station the time at which the first thunder was 

 heard and arrows showing the direction from which the storm came 

 and toward which it moved. Lines are then drawn called isobroutons, 

 or lines of simultaneous appearance of thunder ; these are drawn for tbe 

 full hours, and give a beautiful picture of thi) progress of the thunder- 

 storm. This procedure diifers from that which is customary in France, 

 Norway, &c., where the attempt is made to picture the progress of the 

 center of the storm by taking the mean moment between the first and 

 last thunder. Bezold prefers to draw isobroutons for the last thunder- 

 claps as well as for the first so far as this is possible, but generally 

 finds it difficult to get reliable reports of this last phenomenon. He has 

 also, for the years 1881 and 1882, taken special care to reduce his baro- 

 metric observations and to draw isobars of great accuracy, as a means of 

 studying the relation of a thunderstorm thereto. His conclusions may 

 be summarized as follows: 



(1.) Thunder-storms that do not accompany heavy hurricanes origin- 

 ate when there arise in quiet air notable local differences of tempera- 

 ture and pressure. The isobars drawn for every 5 millimeters show only 

 distortions, but by drawing them more minutely, say for every millime- 

 ter, they show definite centers. These small depressions appear mostly 

 only as partial or auxiliary depressions iu a great area of low pressure. 

 The progression of the thunder-storm in general takes place without 

 reference to the winds immediately attending these small depressions, 

 but in accordance with winds that evidently prevail in the higher regions 

 in connection with the great barometric depression. Especially intense 

 are the thunder-storms that occur in a ridge of high pressure separating 

 two great depressions. 



(2.) If we draw a line inclosing the regions at which in a given instant 

 the first and last thunders are being heard we inclose a space over 

 which electric discharges are then going on. Tiiis is the area of the 

 thunder-storm proper, and, in most cases, this region has the form of a 

 narrow band perpendicular to the direction in which the thunder-storm 

 is moving ; therefore, in general, the storms move with a very broad front 

 and very shallow depth. Cases are at hand where with a front of 300 

 kilometers we have a breadth of only 40. 



295. [This peculiarity was long since remarked by the present writer, 

 who in 1873 called attention to the fact that numerous thunder-storms 

 and even tornadoes frequentl^^ occur nearly simultaneously on what ap- 

 pears to be the advancing edge of a broad area of cool or dry and there- 

 fore dense air ; it is therefore not proper to say that the thunder-storm 

 cools the air, but rather that the thunder storm marks the progress of 

 the advancing cool air, which by uplifting tbe moist air in front of it is 



