THE ANNUAL MEETING. 195 



c — Squalls. On the 19th of August, 1880, at 11 o'clock in the forenoon, 

 a remarkable cloud was seen advancing on Ann Arbor from the west. Its 

 advancing edge had clear-cut, bluish, longitudinal folds, reminding one of 

 folds of calico. The wind was then southwest and running at four miles per 

 hour. At seven minutes past eleven the cloud came over us and we could see 

 ragged edges hanging from its under surface. At the same instant the wind 

 rose to a velocity of 15 miles per hour and at 11 : 17 was as high as 50 miles 

 per hour. Meantime the barometer rose from 28.98 to 29.02 and a few drops 

 of rain fell, at 11:23 the rain began to pour down in torrents and continued 

 to do so for 12 minutes. The thunder had been heard before but the light- 

 ning now became vivid, and the cracking of the electric discharge was frequent 

 among the electric recording instruments. In this interval the barometer 

 reached its highest point, 29.05, and stood there steadily, and the wind was 

 running at the rate of 30 miles per hour ; at 11 : 35 the rain ceased, the barome- 

 ter dropped rapidly and the wind fell. During the storm the wind turned to 

 west and northwest in which direction it remained after the storm passed. 



Here we have an illustration of the squall. In this case it was a thunder- 

 storm, but lightning is not a necessary accompaniment of a squall. This 

 storm was a local center of high pressure from which the wind poured out in 

 all directions. The storm-center passed northeast and we lay to the south of 

 the path of its center. An examination of the diagram for centrifugal winds 

 (1) will show that our succession of winds should have been southwest, west, 

 northwest, which was the succession actually observed. 



Of such squalls Ann Arbor had 15 or 20 during 1880. Their characteristic 

 features are a rapidly advancing, irregular cloud, accompanied by an inde- 

 pendent system of winds having no observable relation to the winds blowing 

 previous or subsequent to the storm. They usually reach us from some 

 western direction, last from 10 minutes to an hour, and are usually accom- 

 panied by rain or hail and lightning. They are most common in the hottest 

 months and the hottest part of the day. 



Some of these storms are very probably centripetal, and it is believed by 

 many that the same storm is at first centrifugal, then centripetal, but the 

 speaker has observed nothing to support either theory. 



d — Tornadoes. The violence of the tornado is very well known. The 

 usual history is as follows : A funnel-shaped spout forms and extends down- 

 ward from a mass of clouds. It is of a dark color, spirally striped and some- 

 times shines with an electric light. Such spouts have been seen to form in the 

 air and extend downward without reaching the earth, and no bad effects have 

 followed from them. Where, however, they have reached the surface they 

 have been found to be accompanied by a system of centripetal winds of 

 extreme velocity and destructiveness. Few structures can stand before them, 

 and the point of the funnel in contact with the earth sweeps a clean path as it 

 goes. It does not always remain in contact with the earth, but sometimes 

 steps over a considerable space. Their progressive motion is nearly east, and 

 their path usually a few rods wide and as many miles long. Tornadoes are 

 most common from Dakota to Texas eastward to Ohio, but they have been 

 known in all the States and territories east of the Kocky Mountains, except 

 perhaps in Florida. Michigan usually escapes, but she has had a severe one 

 in 1880 near Battle Creek, another very destructive one in Port Huron, a few 

 years ago, and another celebrated one at Sault de Ste. Marie. Although they 

 are local storms the conditions producing them may extend over a large area, 

 so that we may hear of several tornadoes in different States within a few hours 



