1829.] 
Of a Portable Barometer. 
315 
The plan is, as I said before, the old one of a tube standing in an open vessel of 
mercury. About six years ago, I sent to Messrs. Dollond & Sons the particulars of 
what I wanted, and the following is a copy of their charges. £ ^ Rs 
2 Brass rods, with divisions from 17 to 31 Inches, 1 3 q' 
2 Brass verniers, J 
12 Barometer Tubes, filled and boiled, 7s. eacli, 4 4 50 
12 Ditto ditto empty, 4s. 2 8 29 
These have been in constant use for five years, and there are yet remaining the two 
brass scales and tubes. So much for the economy. 
The tubes were 33 inches long, and of an internal diameter of T *j inch, (I would 
recommend ^ as preferable.) In consequence of the smallness of their diameter, 
they stood lower than tube, of the full size ; and as I found them differ in this respect, 
I compared each of them with the standard tube which I had, yV inch, internal dia- 
meter. The error being once found, remained pretty constant, hut they were subject 
to a small uncertainty of 01, or 02 inch, owing to the smallnessofthebore occasioning 
sluggishness in the motion of the mercury. For this reason I would recommend 
any person ordering a similar apparatus from England to fix the size at J of an 
inch at least. This will he ample, and yet not too large to make the tubes less por- 
table. Tubes of the size 1 used, might almost be throwndown without breaking, or the 
mercury spilling. , 
The brass scales bad two rings attached by screws, m which the tube was made to 
fit tight by means of a leather collar, with a couple of finger .crews for tightening the 
rings; these finger screws being loosed, the scale was easily moved up and down 
on the tube, and by means of a projecting heel, correctly made to coincide with the 
surface of the mercury in the open cistern, the apparent deviation being doubled by 
reflection, it scarcely required the assistance of a magnifying glass to effect this : with 
the latter, however, the coincidence was easily made quite perfect. 
The following apparatus was used for the purpose of setting it up, and quickly 
making it perpendicular. 
A triangular box of eight inches side, with three woodenfootscrews, affordedstow- 
atre to the several loose a nicies required for setting up and adjustment; and served also 
as a steady and convenient stand on which to fix it during observation. On one side of 
this box was attacheda square pipe, made of deal, ahoutsix inches long, which by means 
ofawooden screw could be made perfectly fixed and steady. Into this square pipe fitted 
the case of the barometer, which was made of thin deal board, inch broad, and put 
together by wooden pins, being strengthened with square wooden dies at the top and 
bottom, about one inch long, on which the sides of the case were screwed, they being 
continued about an inch beyond it. At one extremity it opened at the end by the 
front being cut away for about six inches which piece, being afterwards affixed to a 
similar square diei conveniently fitted into the projecting part, and held the cover 
quite firm. The barometer case, which it is evident was a holloa- prismatic rod, being 
inserted into the square tube affixed to the triangular box, was secured by a wooden 
screw and rendered perfectly firm. The cistern cup, which was a small wedgewood 
ware ’crucible, being now placed on the top of the box, which hadbeen previously 
levelled by the foot screws, was filled with mercury from a bottle, which, as well as 
the cu» had been packed in the triangular box. The straining the mercury from 
the oxyd which invariably collected on its surface in travelling, I suppose from the 
motion, was simply and conveniently effected by means of a notch cut in the lower 
edge of the cork, which being half withdrawn, the mercury passed through the small 
opening in the cork bright and clean as though it had been hltered through a paper 
funnel. The tube, with its brass scale attached, were now taken out of the case, 
and being set up in the mercury, was held in its place by means of a projecting piece 
attached to its wooden case, in the edge of which was cut a notch, something the 
shape of the Greek letter 12, and a small wedge of deal, which was always at hand in 
the waistcoat pocket, being introduced in front of the tube, it remained perfectly 
firm, and allowed of the adjustment of the scale, which was moved up or clown till 
the projecting heel was even with the surface of the mercury. 
I have stated, that the triangular box was levelled by the three foot screws 
previously to setting up the barometer. This was effected by means of a plummet, 
formed of a silk thread with a musket ball attached, which passed through two small 
wooden studs attached to the side of the barometer case, the hole in the lower one 
being made something larger, in order to allow of a judgment as to the thread hang- 
ing freely in the centre. The place of the cistern on the triangular box, being fixed 
originally by experiment, was marked, and the cistern always carefully placed on the 
mark, so that it was at once known, when the plummet hung freely in the centre of 
