[ 2 44 ] 
city being evaporated, the magnetical Power tool: 
place. 
The Cure for this Inconvenience, is to moiften the 
Surface of the Glafs : Even a wet Finger will do it 
immediately and effedually. 
I need not fuggeft, that the fame Quantity of 
Fridion will not at all times have the fame EfFeCt 
upon thefe Glades, any more than it will upon the 
eledrical Tubes; but take the Liberty to hint, that 
I have Reafon to believe that Glafs does, at fome 
times, become in fome degree attractive without any 
Fridion at all 5 and may poflibly be excited by great 
Concuflions in the Air, fuch as Thunder, or the Dif- 
charge of great Ordnance, &c, and, if fo, may thereby 
difturb the Compafs. 
I mult however obferve, that the Mariners Com* 
pafs is much lefs dangeroufly moved by wiping or 
exciting the Glafs than the other; by reafon that the 
excited Part of the Glafs attracts that Part of the Chart 
which lies neared, juft underneath, without giving 
it fo much Verticity, as it does to the other Sort of 
Compafs with a bare Needle. And farther, that the 
deeper, or the farther diftant the Needle hangs below 
the Glafs, the lefs Difturbance it is likely to receive, 
by wiping, rubbing, or otherwife exciting the Cover. 
I (hall make no farther Reflections upon thefe Fads- 
than to obferve, firft, That all the minute, irregular, 
reciprocating Variations which have been obferved 
in the Diredions of dipping and horizontal Needles, 
asmentioned in fome of the Tranfaffians *,.may pro- 
bably- 
* N°. 425. 
