202 PHILOSOPHICAL TRANSACTIONS. [aWNO 1738, 



continues so to do for many days together; you may reasonably expect a fair 

 season for as long a time as it was rising, unless some gales of wind intervene, 

 and especially the s. w. by s. or thereabouts. 



7. When the mercury rises very fast, or falls very fast, neither the fair nor 

 foul weather it forebodes will continue long. 



8. Without knowing how the mercury has stood some little time before, a 

 true judgment cannot be given at all times; for suppose we find it in a rising 

 condition, it will probably be fair; but if it had been higher some hours ago, 

 and fell, there must happen a shower. 



Why the mercury in the diagonal barometer, if it be for fair weather, on 

 tapping the case several times, which jars and makes the tube tremble, will rise 

 at every stroke for several strokes together, and in all sometimes a 10th of an 

 inch, or more, in the perpendicular; may be thus accounted for: 



1. There is a cohesion of the mercury to the tube, which hinders its rising, 

 and such tapping releases that. 



2. But it is observable, that it will rise a little at all times, even when it is in 

 a standing, or even in a falling condition. This may be accounted for thus: 



The mercury and atmosphere are in an equilibrio, and tapping starts and 

 raises the mercury a little in a boiling manner, especially its upper surface, 

 which is seen to leap, or be in a swimming posture; then the pressure of the 

 atmosphere over-balances the remainder of the mercury, and it must rise a 

 little. 



Or such violent jarring puts the mercury in a lateral and upward motion, for 

 downward it cannot go, which takes off its gravity, as the winds lessen the 

 pressure of the air; therefore it must rise a little. 



But then it is observable also, that if the mercury was in a standing condi- 

 tion, or falling, such rising as above, will in a minute return to the same place 

 again ; and even when the mercury is in a rising condition, it will, in that 

 space of time, fall a little part of that it rose by such tapping. 



This barometer has the coruscations, as they were observed in Mr. Patrick's 

 pendant one; for by tapping the case with the finger in a dark place, it will emit 

 several bright flashes, along the empty part of the tube. This is an argument 

 that the vacuum is very pure, and the mercury truly purged. 



Collections from the diary of the weather and barometer, in order to settle rules 

 for foretelling the weather by the barometer. 



Great Storms. 6. . . . night 29.2 



Before them the mercury falls 3 or 4 days, and T. ...night 28.1 



» exceedingly low. 8 noon 27.9 



1734-5, Jan. 4, at night themerc. at 29.92 inc. lower than has been known by ^, and the 

 5. . . . night. 29.66 greatest storm of wind ever heard of in this age. 



