quite cold, this tube is to be fitted into a piece of wood, in which a groove is cut to receive it—the vial part being received into a sort of cup formed in the expanded upper part of the wood. To the lower end of this, the float (also consisting of wood turned on a lathe to the requisite size) is to be attached; and through the middle of it the glass tube must descend to a level with the lower end. The wood and glass are attached by means of wax, and it is necessary to coat the float with the same, to prevent the mercury entering the pores of the wood.
About a foot above the float a cross piece of wood was fixed horizontally, and from either end of this a wooden rod descends about four feet—these rods ought to admit of being easily and firmly attached to, or removed from, the rest of the apparatus. They support the wooden shelf on which to place the weights which act as ballast, and it ought to be broad to admit of moving the weights into the position that will cause the whole instrument to float perpendicularly. The vial ought to be filled with mercury before the tube is cemented to it—however, this is not essential, as the mercury can easily be introduced down the tube—care must be taken, however, to incline the tube as the mercury rises in this, otherwise the great hydrostatic pressure may burst the vial. The cistern which holds the mercury may easily be made out of a common beer bottle, by cutting off the top narrow portion with a red hot iron. The upper part of the float is considerably smaller than the lower, with the view to admit of the indications being magnified.
Compared hourly with a common barometer, the two rose and fell simultaneously, but not regularly. This of course arose from the unequal diameter of the vial, and the irregular contour of the float—it was intended that the indications should be magnified five times—but at some hours the indications were greater, at others less, than those dimensions—this irregularity of course arose from the cause just mentioned—but, as already stated, all I contemplated, was merely to ascertain whether a barometer on this principle will work, which was proved most satisfactorily.
This instrument, compared with the balance variety, recommends itself for its great comparative simplicity of construction.
The plan of bringing the centre of gravity below the float, would be advantageously introduced in the variety of barometer shown by fig. 2, plate 15, in the 14th Number of this Journal.
Hoonsoor, 5th October, 1837.
