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The Restoration of a Nineteenth Century FitzRoy Barometer.


Admiral FitzRoy has a deservedly honoured name in the history of barometers and weather observation and forecasting. Indeed, it may be said that he is the father of the science of Meteorology as we know it today. His wide experience as a sea captain and global explorer, his creative intellect and his keen powers of observation and measurement combined with a commitment to the welfare of seafarers and fishermen, led him to realise and exploit the measurement of barometric pressure for the prediction of storms for the preservation of life and the protection of shipping.

FitzRoy pioneered the interpretation of barometric observations and did much to equip sea ports and fishing harbours with the necessary instruments to make those interpretations possible. The history of this achievement has been well recorded by Collins of Barometer World, (to whose book, FitzRoy and his barometers, ISBN-13:978-0-948382-14-7, the interested reader is referred) and it is not surprising that such a career gave rise to Fitzroy's name being attached to thousands of barometers. Some such instruments were those he enabled to be distributed to coastal locations, some were of the same design and quality, but sold and bought on the open market, and some were inscribed with FitzRoy's name and carried his "remarks", but otherwise had little to do with him. Many of these barometers were manufactured throughout the latter part of the Nineteenth Century and into the Twentieth, and, being mass-produced, were not always of very good quality, and certainly not very accurate. Collins points out that the craftsmen who made them may have had little technical understanding of what they were making: the length of the barometer tube, and indeed, the whole instrument can be somewhat too short and the scale necessarily positioned to give an inaccurate reading.

The instrument which is the subject of the restoration described here is of such a class. It looks quite presentable, but the distance from the base of the glass mercury cistern to the scale is a half of one inch too short, so that the instrument cannot be properly set up for use at sea level. This disadvantage was alleviated somewhat by the fact that it is intended for present use at an altitude of 580 feet above sea level, which made it possible to adjust the mercury level in the cistern so the instrument the corrected pressure at sea level.

1. The Case.

Figure 1 shows the overall instrument. The glass front slides in from the bottom along grooves in the side plates and is retained by the base of the instrument via two woodscrews securing the base to the case. Removing these screws allows the base and glass to be removed. The glass front had a semicircular top edge which fitted behind the carved top. In the present case the glass was intact and so only required cleaning. It was, however, thin Victorian glass and showed some minor casting defects, so it was handled with considerable care.
The storm glass, which was empty of fluid, the thermometer which was intact, and the barometer tube which was devoid of mercury were all removed by carefully prising out the brass pins used to secure them. These pins were polished and lacquered and set aside. With the instrument components removed, the condition of the scales and interior woodwork could be examined. The scale behind the stormglass had been very adversely affected by the storm glass contents which had leaked out all over it, reducing it to a friable, powdery condition from which it could not be retrieved. It was carefully copied and a replacement made. The main scale was in better condition and certainly worth preserving, but was very darkened by age and not very easy to read. It was resolved to leave it in place untouched, and to install a replacement in such a way as to preserve the original (i.e. without glue or any such thing) . It turned out rather easy to do this as there were thin wooden fillets lining the frame which could be used conveniently to secure the new scale.

2. The Brass Components.

These comprised the pins, the saddles securing the glass parts, and the index mechanisms on each side for recording readings. The pins and the saddles were cleaned, polished and lacquered. The index mechanisms needed some further work. Figure 3 shows how they function. The pinion, shown removed from its slot in Fig 3, was a brass 10-leaf clock pinion, filed to a 1.75mm square section at the upper end and turned to form a blunt pivot at the lower end. It is introduced into its slot from the rear of the instrument and retained there by a thin steel plate perforated with a hole to fit the blunt pivot and having two other holes through which screws held the plate to the case. In the subject instrument, these plates had become completely corroded and had broken with the consequent loss of the right hand pinion altogether. A replacement was fashioned from 10-leaf pinion blank obtained from skeletonclocks. Two replacement steel plates were fashioned for the rear of the instrument and these were made in 0.5mm stainless steel sheet to prevent a repeat of the problem.

3. The Storm Glass

The author is entirely unconvinced that the storm glass has any reliable merit for weather observations and the most that can be said for it is that it represents a very crude thermometer. However, it is a historical component of the subject barometer and was restored using the mixture advocated by Negretti and Zambra. This consists of 2.5gm of Potassium Nitrate and 2.5 grams of Ammonium Chloride dissolved together in 33ml distilled water. To this solution is carefully added a solution of 10gms of natural Camphor in ethanol. According to some reports, natural Camphor "works" better than synthetic because the latter contains borneol which has an adverse effect. The mixing of the solutions produces an immediate white precipitate of Camphor which redissolves on warming. The storm glass tube shown in Figure 4, was washed with detergent and filled with concentrated nitric acid and allowed to stand overnight. It was then rinsed with distilled water, allowed to drain and filled with the warmed storm glass mixture. It was securely corked. As the mixture cooled a voluminous white precipitate formed filling most of the tube and this slowly subsided to occupy less than half the tube over a period of days.

4. The Thermometer

The Thermometer is shown in Figure 5. It is an attractive instrument, mounted on hardwood (beech?) with a grained ivory scale The bulb is pear shaped. Apart from cleaning and polishing the saddles, no other restorative work was carried out.

5. The Barometer Tube

The Barometer tube was cleaned by filling it with concentrated nitric acid to remove all Mercury and Mercury oxide residues using the apparatus in Figure 6. A teflon tube 1mm ID and 1.6mm OD was threaded to the far end of the barometer tube which was then clamped vertically upside down. The teflon tube was attached to a glass syringe filled with nitric acid and the barometer tube completely filled and left to stand overnight. The nitric acid was then withdrawn and the barometer tube set upright and a further quantity of nitric acid allowed to remain for some hours in the cistern and U tube. The acid was then withdrawn and the barometer tube washed thoroughly with water and then rinsed with acetone. It was then pumped down to 50 microns of mercury pressure by means of a good rotary vacuum pump and left under vacuum for four hours.

Figure 7 shows the apparatus used for filling the barometer tube with mercury. Again a teflon tube 1mm ID & 1.6mm OD was used and threaded to the top of the barometer tube, the latter clamped upright. A glass stopcock sealed to a borosilicate syringe barrel was used to connect the free end of the teflon tube to the vacuum line. Clean, doubly vacuum-distilled mercury was poured into the cistern and the stopcock opened to the vacuum line. The mercury was pulled up the barometer tube to the barometric height. Thereupon the barometer was pumped continually for 3 hours to achieve the best possible Torricellian vacuum in the finished instrument. The stopcock was then closed and immediately the teflon tube was withdrawn from the barometer tube, filling the syringe with mercury in the process. The barometer tube was checked for the correct height at the prevailing atmospheric pressure.


The installation of the barometer tube, the securing of it with the brass saddles and the final installation of the glass front and base was straightforward but great care was needed to maintain sufficient vertical orientation of the barometer tube to prevent any compromise of its vacuum. Rapid movements had to be avoided to prevent the mercury striking the top of the barometer tube with any significant force.

The barometer reading was adjusted to that of sea level by adding or removing mercury from the cistern.

The instrument was provided with two-hole brass wall plate by which it could be suspended on a wall a height convenient for reading the pressure.









Figure 2. Detail of the Scale of the Restored Instrument


Figure 3 The Rack and Pinion index mechanism.

Figure 4. The Storm Glass mounted in the case

Figure 5 The Thermometer

Figure 7. Apparatus used to fill the Barometer Tube with Mercury.