REPORT OP THE COUNCIL. 
XXXI 
being covered with strong sheep-skin leather firmly fixed by abutting flanges. 
This leather has been found to be sufficiently porous to permit the free 
action of the air through it, but not to allow the mercury to pass without 
considerable pressure, to which it can never from the construction of the 
instrument be subjected. The cistern is filled with mercury to such a height 
that it can never under any circumstances of temperature or pressure be full, 
but always sufficiently so to prevent the lower end of the tube being ever 
exposed either during carriage or when in use. This renders any adjust¬ 
ment of the instrument when being mouuted for observation unnecessary. 
The diameter of the cistern is about 1*3 inch. The barometer tube Is pro¬ 
tected by a cylindrical case of brass, which is screwed firmly to the upper 
portion of the iron cistern. The graduation is made on this bra**> tube and 
the vernier moved by a rack and pinion, the index being adjusted t<> thu top 
of the mercurial column by shutting off the light, as is commonly done in 
standard and other good barometers: the vernier reads to 0 002 inch. The 
correction for the relative capacities of the tube and cistern (which is usually 
applied as a numerical correction varying with the height) is in these instru¬ 
ments included in the gradualiou of the scale ; the scale being shortened by 
the amount of the correction, but divided so as to represent the true measure¬ 
ments. The correction for capacity is obtained by computation from care¬ 
fully measured diameters of the tube and cistern. The zero-point of the 
scale is determined by comparison with a standard barometer. A thermo¬ 
meter, whose scale is divided on its stem, and having its bulb within the 
encasing brass tube, gives the temperature of the mercury. In making an 
observation, it is only necessary to set and read off the vernier, and to not<- 
the height of the thermometer. The instrument presents much the appear¬ 
ance of a mountain barometer: it is suspended in gimbals from a point a 
little above the middle of the tube, the rack motion being close to the point 
of suspension, so that the hand may rest on the supporting arm. 1 he support¬ 
ing arm is flat, of hammered brass, thin enough to give the elasticity neces¬ 
sary to counteract sudden jars, and is equivalent to the spring gimbals usually 
employed, while much simpler in its construction.” 
The annexed copies of the correspondence with the Hydrographer of the 
Admiralty and with the Hoard of Trade, will explain the proceedings of your 
Committee as to the proposed supply of these instruments for the use of Her 
Majesty’s Navy, as also for the Mercantile Marine of this country. Your 
Committee have also communicated with Lieut. Maury, by whose directions 
the fifty barometers were ordered from Mr. Adie by Mr. Stevens, for the use 
ot the United States Navy, all of which are to be verified at the Observatory. 
Standard Barometer .—Your Committee have devoted much attention to 
the completion of this instrument, but many casualties have occurred during 
the progress of its erection. On the 13th July last Mr. Negretti succeeded 
in boiling and erecting a tube of owe inch internal diameter. Considerable 
changes were found necessary in the original mounting of the barometer and 
cathetometer. These have been .satisfactorily executed at the Observatory, 
but the instrument still requires alteration to render its performance, perfect. 
The scale ol the cathetometer has been compared with the Kew standard 
scale, and found not to require any correction when laid horizontally, 
uhen the two scales are compared by observing their relative lengths ver¬ 
tically by the telescope of the cathetometer, a perceptible error is shown, 
involving a correction to the measurements of the cathetometer of -f 0003 
inch in a space ot 30 inches. This anomalous error is, it is conceived, due 
to faulty construction in the cathetometer, chiefly arising from the difficulty 
of making the instrument accurate on the design adopted. It is, therefore, 
