OP ARTS AND SCIENCES. 21 



This storm was visible from Cambridge as a great mass of boilin<r 

 cumulus clouds overlain with fiat, outspreading cirrus; their angular 

 altitude was estimated at 30°, giving a height of five or six miles. 

 A third storm, or group of showers, less defined than the others, 

 appeared first on the Connecticut River in Massachusetts about \b^ to 

 1 &", and gave moderate showers to various stations on its way to the 

 coast, at and north of Boston, where it arrived at 21'' or later. The 

 first two storms here described were much like, but smaller than the 

 noon thuuder-squall of July 21, that is described in detail farther on. 

 Very numerous and careful observations are needed for the full 

 definition of such storms. 



June 17-21. Another period without thunder-storms now appears. 

 On the morning of the 17th, a large area of high pressure appeared 

 over the Mississippi valley; on the 18th, it was on the Ohio valley ; 

 on the 19th and 20th, on the Middle Atlantic coast. On the 21st, an 

 ill-defined area of low pressure was central on Lake Michigan. All 

 these days were fair and warm in New England, and on the 20th the 

 mean maximum temjierature was 85°. 



June 22. The low pressure area was central north of Montreal in 

 the morning, with barometer down to 29.40 ; the isobars were strongly 

 V-shaped, with the axis extending down the Hudson valley ; but in 

 spite of this apparently favorable condition for the collision of warm 

 southwest and cool northwest winds, New England had only light 

 showers, and among these there were several without thunder. No 

 well-developed thunder-storm appeared. The temperature was mod- 

 erate (mean max. 77°). 



June 23-25. This period is a repetition of that from June 17 to 

 21 ; an anticyclonic area moving from the Mississippi valley to the- 

 sea-coast, with fair weather in New England ; the mean maximum 

 temperature was only 69° on the 23d, but rose to 84° on the 25th. 



June 26. A number of stations in southern New Hampshire, and 

 a few in southwestern Maine, report moderate thunder-storms on this 

 date at noon, in the afternoon, and at night. Stations were wanting 

 farther north, so it cannot be said how far the storms extended in 

 that direction ; but they were definitely absent in the south, in spite 

 of continued warm weather (mean max. 83°). These storms are less 

 distinctly dependent on cyclonic conditions than any of their pre- 

 decessors, as there was no well-defined low pressure centre within the 

 limits of the weather map at the time; a high pressure area was mov- 

 ing eastward from the Middle Atlantic States. 



June 27. An area of slightly diminished pressure appeared on this 



