22(5 PHILOSOPHICAL TRANSACTIONS. [aNNO I698. 



height of 82 feet the mercury fell Vrr of an inch, and at about 164 feet -^. 

 I also repeated my experiment, by ascending and descending quicker. At 

 both which times, my observations agreed exactly with the first trial. From 

 whence I conclude that at every 82 feet height, or thereabouts, the mercury 

 will descend Vt of an inch. 



Considering that there is a difference of 8 feet between Mr. Halley's ob- 

 servations and mine, I am inclined to think, that a higher ascent than 82 

 feet is necessary to cause the mercury to descend -jV*, the higher we are in 

 the atmosphere. 



To make a portable barometer, provide a strong glass tube ; let its head be 

 pinched at about an inch from the top, so as to make a narrow neck, whose 

 orifice shall be almost as wide as a straw. This will check much the blow of 

 the mercury against the top. The bottom of the tube is to be grourd aslant 

 near half an inch, that the bottom of the tube touching the bottom of the 

 cistern, its orifice may lie about the middle of the mercury in the cistern; 

 which will prevent the air getting into the tube, because the mercury is always 

 about the mouth of the tube. The cistern must be made wide, either of glass, 

 or close-grained wood ; round the brim of which, on the outside, must be a 

 notch to tie on the leather that is to cover it. When the tube is filled, cleared 

 of air, and plunged into the cistern nearly full of mercury, inclose the mercury 

 with soft leather, tied very fast round the tube near the bottom ; which being 

 spread over the cistern, tie it round that also : the tube and cistern, thus con- 

 joined with leather, must be lodged in a case, made very fit to receive both, 

 where they must lie very firm. Through the case let 3 or 4 holes be bored, to 

 let the air in freely to the leather that covers the cistern, which, lying close 

 against the holes, will firmly enough keep the mercury from running out at 

 them. The whole instrument, thus prepared, must be suspended by the top : 

 for which purpose a tripos may be best, whose legs open and shut by joints 

 at the top. The weather-plates are to be put upon the frame, by setting them 

 to the same height, at which the mercury stands in a common barometer. 



On the Effects of a great Storm of Thunder and Lightning at Everdon in North- 

 amptonshire. By Dr. Wallis. N° 236, p. 5. 



In harvest time, on the 27th of July, 169], in Everdon-Fiekl, near Daven- 

 try in Northamptonshire, several persons were at work reaping corn. The 



* The altitude answering to the first tentli of .nn inch is found to be various, according to the 

 temperature of the air : in the mean temperature of 60°, that altitude is \)'i feet, or 31 yards. And 

 for greater altitudes, it is still more. 



