( n 7 ) 
lighter than Mercury , would afeend about times 
as much as the Mercury in the Barometer fell. This, 
befides that the Spirit would dilate and contrad itfelf 
with heat and cold, had the inconvenience of the for- 
mer, not to be eafily removed without great danger 
of diforder and breaking, by reafon of the fmallnefs 
of the Tube in which the Spirit was to rife and fail. 
This was fucceeded by Dr. Hook' s Marine Barometer, 
made of two Thermometers, the one the common 
feal’d Weather-glafs, having no Communication with 
the outward Air, wherein the temper as to Heat and 
Cold was (hewn by the fwelling or fhrinking of the 
included Spirit ; the other the old Thermometer made 
with an inverted Bolt-head, in whofe globular part 
was included Air fomewhat rarer than the ambient, 
fo as to make the Liquor which was to rife and fall in 
thelhank of the Bolt-head, always to Band above the 
furface of the Stagnum, into which its end was immerfed. 
This fhew’d the heat of the Air by its own Dilatation ; 
but at the fame time, the different preflure of the 
Atmofphere mixed with it, fo that the graduation of 
thefe two Thermometers being adjufted to any given 
height of the Mercury, they would at all times when 
the Mercury was at that height, both fhew the fame de- 
gree of heat : But at other times when the weight of 
the Air was different, that difference would fhew it 
felf by the difagreement of the degree of Heat (hewed 
by them. This will be better underftood from N° 269, 
of the Tranfatfioxs, wherein I have deferibed thislnftru- 
ment at large. This, tho’ of admirable ufe at Sea, to 
gi ve timely notice of approaching bad Weather, labours 
under the Objedion that it fuppofes the Concave of 
the Tubes of the Thermometers to be Cylinders, or 
of equal Diameters throughout j and alfo that cn 
account of Heat and Cold the Air and Spirit have a 
R 2 propor- 
