Royal Society. 241 
the bottom of the tube into a broad ciftern of mercury, and 
then fixed both the tube and ciftern together in a wire-cafe, or 
frame 5 on the top he left an eye in the wire, to fufpend the 
whole barometer on a ih'ing, that it might hang pendulous, 
which is abfolutely neceflTary j becaufe, if the ciftern be deeper 
on one fide than another, or if the tube hang more towards one 
fide than the other, it will caufe a great, and erroneous varia- 
tion in the mercury above, according as the tube ftands per- 
pendicular or not: His inftrument being thus very nicely 
prepared, Mr. 'Derham marked exafily the height of the 
quickfilver on two narrow labels of paper, pafted on each fide 
the tube, both at the bottom, and in his afcent up the monu- 
ment 5 he meafured the differences of the height of the mer- 
cury, with a decimal inch fcale of thin brafs, and the quantity 
of his afcent with a Gunter^ chain, becaufe a ftring would 
ftretch 5 and by the niceft obfervation he could make, be found 
that at the height of 82 foot, the mercury fell fo of an inch, 
and about 154 foot fo ; and repeating the fame experiment, 
his obfervations agreed exactly with the firft trial j from whence 
he concludes, that at every 82 foot height, or thereabouts, the 
mercury will defcend lo of an inch. 
In order to make a portable barometer, provide a ftrong glafs 
tube, let its head be pinched at about an inch from the top, lb 
as to form a narrow neck, whofe orifice /liall be as big alnioft 
as aftrawj this will check much the blow of the mercury 
againft the top ; the bottom of the tube is to be ground a-ilanc 
near half an inch, that the bottom of the tube touching the 
bottom of the ciftern, its orifice may lie about the middle of 
the mercury in the ciftern, which will prevent the air's getting 
into the tube, becaufe the mercury is always about the mouth 
of the tube j the ciftern muft be made void, either of glafs, or 
clofe grained wood 5 round whofe brim externally, there muft 
be a notch to tie on the leather, that is to cover it 5 when the 
tube is filled, cleared of air, and plunged into the ciftern 
almoft full of mercury, enclofe the mercury with gentle leather, 
tied very faft round the tube near the bottom, which being 
fp read over the ciftern, tie it round that alfo^ the tube and 
ciftern, thus faftcned with leather, muft be lodged in a cafe, 
made fit for receiving both, where they muft lie very firm 5 let 
three or four holes be bored thro' the cafe, to let the air in 
freely to the leather that covers the ciftern, which, lying clofe 
againft the holes, will firmly enough keep the mercury from 
running out at them. 
Vol. III. H h To 
