029 



BARO'METER. 



BARO'METER. 



930 



mounted, but did not work satisfactorily ; the adhesion of the mercury 

 to the glass was such, that when the barometer was set up the convexity 

 of the top of the column was destroyed, and the surface even became 

 concave ; after a few days' working, rings of dirt or other impurity 

 were formed on the glass near the top of the column, and they continued 

 to increase so as to interfere with observation. Nevertheless, care had 

 previously been taken to render the mercury chemically pure ; and, 

 when examined by Professor Miller of King's College, no impurity was 

 detected in it. The great heat to which the glass is exposed during 

 the boiling, applied as it is in an irregular and unequal manner, pro- 

 bably has the effect of reducing the glass to its unannealed condition, 

 in which the particles are held in a state of unstable eqxiilibrium (as in 

 the Bologna phial), so as to be liable to be overturned from a very 

 slight cause. The fouling of the mercury seems to have arisen from 

 impurities in the glass tubes; for when greater care was taken by the 

 glass-blower, the effect on the mercury was decidedly diminished ; and 

 when the tubes, before being filled, were thoroughly cleansed by 

 sponging them with whiting and spirits of wine, a satisfactory result 

 was obtained ; for after a year's trial it was found that the top of the 

 column presented a good convexity in all states of the barometer, with 

 only a very slight trace of dirt. In this case the standard barometer 

 was filled, not by boiling, but by a method devised by Mr. Welsh of 

 the Kew Observatory, and described by him in a paper communicated 

 to the Royal Society in 1856. 



The tube to be filled was first prepared in the following manner : 

 To its upper end was attached a bent capillary tube, A, D, E, F, 

 with its bore much contracted at the apex, D, with a small bulb at E, 

 and drawn out to a fine point at F, where it was sealed. To the lower 

 end of the large tube was attached a smaller txibe B, c, o, '3 inch bore 

 and 10 inches in length, and to this was added the short capillary 

 tube, o n, with a bulb at o. The end H of the capillary tube was next 

 connected with a good air-pump, and the air was very slowly extracted 



while the whole tube was being strongly heated by passing a large 

 spirit-flame along it. 'When the air had been as far as possible extracted 

 in this way, and while the air-pump was still being worked, and the 

 heat still applied, the capillary tube o n was sealed at I by means of a 

 blowpipe flame. When the tube had cooled it was placed at a small 

 inclination with the end P in a vessel of well-boiled mercury, when, the 

 point F being broken off under the mercury, the metal rose in the tube 

 by atmospheric pressure, and continued to rise until the bulb at o was 

 more than half-filled, the remaining space being occupied by the air 

 which the pump had not extracted. The pressure of this residual air 

 was calculated to be less than "05 inch. The basin of mercury was 

 then withdrawn from beneath the tube, leaving the point P exposed, 

 while the small tube remained quite filled with mercury. The blow- 

 pipe was then applied at F, and the opening sealed. When the glass at 

 F had cooled, the tube was placed erect, the mercury separating at the 

 contracted part, n, leaving the tube from D to F filled or very nearly 

 so, and from D to A empty. The tube was next sealed at K, and the 

 portion K D E P removed. Lastly, the lower end of the tube was 

 broken under mercury at c, leaving about an inch of the siphon. 

 This barometer tube has an internal diameter of I'l inch. It is sup- 

 ported over a glass cistern, in a strong brass frame, secured by brackets 

 to the wall of the old mural quadrant of the Kew Observatory, the 

 height of the mercury being measured by a cathetometer fixed to the 

 same wall at a distance of five feet. A conical point at the lower end 

 of a short rod of steel is adjusted by a screw to the surface of the 

 mercury in the cistern, as shown in the figure. At the upper end of 

 the steel rod, and above the level of the glass cistern, is a fine mark, 

 the distance of which from the conical point is 3 J 515 inches. In making 

 an observation, the lower point is adjusted to exact contact with the 

 mercury in the cistern ; the telescope of the cathetometer is then levelled, 

 and its horizontal wire made to bisect the mark on the upper end of 

 the steel rod ; the scale reading of the cathetometer being noted, the 

 telescope is then raised, again levelled, and the wire made a tangent to 

 the surface of the mercury in the tube, the cathetometer scale being 

 again observed ; the difference between the two readings of the catheto- 

 meter, added to the length of the steel rod, is the column of mercury. 

 To prevent light from being reflected into the telescope from the 

 surface of the mercury in the txibe, a moveable screen, the upper part 

 af which i black and the lower part of oil-paper, in so adjured as to 



AT!T5< AM) KCf. IXV. VOf,. T. 



shut off all light that comes from a higher level than the top of the 

 mercury, by which means the surface of the column presents in the 

 telescope a well-defined dark outline. A window behind the barometer 

 illuminates the paper screen by day, and a lamp by night. A ther- 

 mometer with its bulb in the mercury of the cistern, gives its tempe- 

 rature, and the brass scale of the cathetometer is corrected by means 

 of the usual tables. 



This fine instrument is used as the standard for correcting the 

 barometers supplied by the Board of Trade to captains of merchant 

 ships, willing to make observations at sea, and also to various meteoro- 

 logical stations in the colonies.* The method of testing is described 

 by Mr. Welsh, ' Phil. Trans. 1856 '; and also in the ' first number of 

 ' Meteorological Papers, published by the authority of the Board of 

 Trade, 1857,' from which we gather the following particulars : In the 

 barometers used at sea, the usual method of adjusting the mercury in 

 the cistern to the zero point or the reverse, previous to an observation, 

 cannot be adopted on account of the motion of the ship, which motion 

 would produce in an ordinary barometer an oscillation of the mercurial 

 column known by the name of pumping. To prevent this, part of the 

 tube of the marine barometer must be very 

 much contracted, one consequence of this 

 construction being that the index correction 

 varies through the range of scale readings in 

 proportion to the difference of capacity be- 

 tween the cistern and the tube. To find the 

 index correction for a land barometer, com- 

 parison with a standard at any part of the 

 scale at which the mercury may happen to be, 

 is usually considered sufficient; but to test 

 the marine barometer it is necessary to find 

 the correction for scale readings at about 

 each half-inch throughout the range of atmo- 

 spheric pressure to which it may be exposed. 

 Hence it is necessary to have means at com- 

 mand for changing the pressure of the 

 atmosphere on the surface of the mercury in 

 the cistern. For this purpose the barometers 

 to be tested are placed with a standard in an 

 air-tight chamber of iron from which a por- 

 tion of the air can be pumped out, so as to 

 diminish the pressure, or air can be forced 

 in so as to increase the pressure. The tube 

 of the standard is contracted as in the marine 

 barometer, but provision is made for adjusting 

 the mercury in its cistern to the zero point. 

 The upper part of the air-chamber contains 

 glass windows through which the scales of the 

 thermometers are visible, but as the verniers 

 cannot be got at, the height of the mercury 

 is read by means of the fixed vertical scale 

 and cathetometer already referred to. The 

 fixed scale is 5 or 6 feet from the chamber, 

 and its divisions exactly correspond with those on the tube of the 

 standard : a vernier and telescope slide on the scale by means of a rack 

 and pinion ; the telescope has two horizontal wires, one fixed and the 

 other moveable by a micrometer screw, so that the difference between 

 the height of the column of mercury, and the nearest division on the 

 scale of the standard, and also of all the other barometers placed by 

 the side of it for comparison, can be measured either with the vertical 

 scales and vernier or the micrometer wire. In this way the barometers 

 are usually tested at every half-inch, from 27'5 to 31 inches. In some 

 barometers the errors are so large as to read half-au-iueh and upwards 

 too high, while others read as much too low. In some cases those 

 which were correct in some part of the scale, were found to be from 

 half-an-inch to an inch wrong in other parts. Indeed the marine baro- 

 meter, until lately in common use, was so faulty in construction that 

 the cistern was not large enough to contain all the mercury, which 

 descended in a low atmospheric pressure, so that in some cases it was 

 found that 29 inches was the minimum pressure that the barometer 

 could indicate. 



The marine barometer recently introduced on the recommendation 

 of the Kew Committee, is a superior instrument. The diameter of the 

 cistern is about 1 J inch, and that of the tube about ^ inch. The scale, 

 instead of being divided into inches in the usual way, is shortened in 

 the proportion of about P 04 of an inch for every inch, which prevents 

 the necessity for applying a correction for difference of capacity between 

 the cistern and the tube. To check the pumping of the mercury the 

 tubes are so contracted through a few inches, that when first suspended 

 the mercury may be twenty minutes in falling from the top of the tube 

 to its proper level. This contraction of the tube causes the marine 



* In tlie year 1853, a conference of maritime nations was held at Brussels on 

 the subject of meteorology at sea. The report of this conference was laid 

 before Parliament, which voted a sum of money for the purchase of instiuments, 

 and the discussion of observations under the superintendence of the Board of 

 Trade. Arrangements were then made in accordance with the views of the 

 Royal Society and the British Association for the Advancement of Science, for 

 the supply of instruments properly tested. 



u o 



