540 PROFESSOR DANIELL ON THE WATER-BAROMETER ERECTED 
but the column of water probably in this, as in all the other instances which I 
have met with, was raised by the imperfect rarefaction of the air in the tube 
above it, or by filling with water a metallic tube, of sufficient length, cemented 
to a glass one at its upper extremity, and fitted with a stop-cock at each end; 
so that when full the upper one might be closed and the lower opened, when 
the water would fall till it afforded an equipoise -to the pressure of the atmo¬ 
sphere. The imperfections of such an instrument, it is quite clear, would 
render it totally unfit for the delicate investigations required in the present 
state of science ; as, to render the observations of any value, it is absolutely 
necessary that the water should be thoroughly purged of air, by boiling, and 
its insinuation or reabsorption effectually guarded against. I was convinced 
that the only chance of securing these two necessary ends, was to form the 
whole length of tube of one piece of glass, and to boil the water in it, as is 
done with mercury in the common barometer. The practical difficulties which 
opposed themselves to such a construction long appeared to me insurmount¬ 
able; but I at length contrived a plan for the purpose, which, having been 
honoured with the approval of the late Meteorological Committee of this 
Society, was ordered to be carried -into execution by the President and Council. 
The first object was to procure a glass tube of the proper diameter, and of 
sufficient length for the purpose. Messrs. Pellatt and Co., of the Falcon 
Glass House, very obligingly consented, upon application, to permit the trial 
to be made at their works; such an undertaking never having been before 
attempted. Accordingly, a very strong packing-case was prepared of one 
inch-and a-half deal, forty feet long, five inches wide, and four inches deep, 
inside measure; with a cover of the same thickness to screw down upon it. 
This was carried to the glass-house, and being laid in the yard with its cover 
off, small pieces of wood were placed across its bottom, at about one-foot in¬ 
tervals. The only instructions given to the workman were to make a tube of 
the length of the box, which should not be less than half an inch internal 
diameter, and as equal throughout its length as possible; and the manual 
dexterity with which he proceeded to effect this was well worthy of admiration. 
Having collected the glass at the end of his tube, and blown the cavity, a boy 
attached another iron with a small lump of hot glass to the opposite extremity 
of the mass, and drew the tube out by walking away to the required distance. 
