Bo is Defcription of a Barometers 
vier, the mercury falls in the tube DC, and when lighter 
it rifes. The range of the feale is about 3 inches, being 
equal to that of a common barometer of the beft conftruc- 
tion, which has a bafon with a yery broad furface. This 
inftrument moves in a direction contrary to that of the com» 
mon barometer, the one rifing while the other fails. 
Fig. 4 reprefents the tube DC, &c. on a larger feale. 
This part of the inftrument is conftruéted exactly in the 
fame way as the fcale, float, &c. of the thermometer de- 
fcribed in the article immediately preceding the prefent. F 
is the float, having the float-wire fixed to it, terminating in 
a knee at a right angle between the indexes LL, where it 
embraces a very fmall wire ftretched along the feale, and 
thereby raifes or lowers them as the mercury rifes or falla 
in the tube DC. 
_ The barometer is prepared for obfervation, by bringing 
down the one and raifing the other index till both touch the 
knee of the float-wire. When next obferved, the upper 
index will point out the greateft depreffion of the mercury - 
—or lightnefs of the atmofphere ; and the lower the greateft 
rife of the. mercury—or weight of the atmofphere fince the 
feale was prepared. 
By this means, the variations of the atmofphere are much 
more truly pointed out than by the common barometer ; 
for it often happens, that, during tempeftuous weather,,or 
before it, the mercury both rifes and falls within a few 
hours, or during the night-time; which variations cannot 
be noticed by any of the barometers now in ufe. The 
Judden fall and rife, or even the rife and fall of the mercury, 
“always denote an extraordinary agitation in the atmofphere. 
By a common barometer the mercury may be at the fame 
height in the morning that it was the night before; which 
leads to a conclufion, that as there has been no agitation 
of the mercury, there will be calm or fettled weather: but 
this new barometer will often fhew in fuch cafes, that the 
one 
