574 
PROFESSOR DAN I ELL ON THE WATER-BAROMETER. 
are many considerable oscillations of the aqueous column which are totally lost 
in the mercurial, and will prove that much curious information with regard to 
atmospheric changes might he derived from a long-continued series of such 
observations. 
The most important result, however, and that which alone would have amply 
repaid all the labour expended upon the subject, is the fact pointed out by 
the observations of Mr. Hudson, that the water-barometer precedes by one 
hour the barometer of half-inch bore, and the latter the mountain barometer 
of 0T5-inch bore by the same interval, in their indications of the horary 
oscillations ; showing that while philosophers are disputing about the hours 
of the maxima and minima, much depends upon the construction of the in- 
struments observed ; and proving the necessity, which I long ago pointed out, 
of making these delicate observations with instruments which have been com- 
pared with accurate and known standards. This comparative sluggishness 
of the mercurial barometer, when compared with the water, also proves that 
the difference between the two, when reduced by calculation of their specific 
gravities to the same expression, can only at times approximative^ determine 
the elasticity of the included vapour ; and that such determination must always 
be liable to a small error from this circumstance. 
Should the Council of the Society hereafter come to the conclusion that 
there is enough of interest in the subject to induce them to prosecute it further, 
I am of opinion that the water-barometer might be reboiled and resealed with- 
out much risk ; and I think that if a stratum of oil of four or five inches depth 
were afterwards poured upon the surface of the water, there would be little 
risk of the air again insinuating itself within it. 
