Fortin barometer
Date: circa 1900
Inventory Number: 2000-1-0027
Classification: Barometer
Dimensions:137.4 x 24.3 x 32.4 cm (54 1/8 x 9 9/16 x 12 3/4 in.)
DescriptionA mercury Fortin barometer, suspended by square brass ring from hook near roof of wood and glass case; held in place by three screws in metal ring, which are attached near the bottom of the case. The case opens in front by hinges. The glass Mercury tube is supported by outer metal casing. There is a cistern of mercury at the bottom, with a brass knob underneath for adjusting the level of mercury, the standard heigh of which is indicated by an ivory point in top left side of cistern. An internal leather bag is contains the mercury and allows for adjustment.
There is a white strip of paper on case wall behind cistern to reflect light onto the mercury. The mercury level in the tube is measured by a ring index, adjusted by brass knob on side of metal tube. The sliding vernier has a scale in inches on one side and centimeters on the other.
There is also a thermometer on the instrument, below the barometric scale, with a scale in both °F and °C.
Scales:
On temperature scale: F, C
Scale reads from -10-120 °F, and 20-50 °C
The barometric scale runs from 25 1/2-33 inches, and 65-83 cm.
Signedon cistern: J & H.J. GREEN / NEW YORK
FunctionA portable device for measuring atmospheric pressure. During transportation, the bottom screw would push a leather bag underneath the cistern up, at the same time as the instrument was being gently tilted so that the mercury would flow into the tube. This had to be done quite carefully since the mercury was capable of breaking the tube if it slammed into it with too much force. When the leather bag was fully adjusted, the mercury would essentially be trapped in the tube, and could be carefully transported upside-down.
During use, the screw would be adjusted so that the mercury flowed back into the cistern, and the instrument would be turned upright again. The small brass hoop at the top would have been used to hang the instrument from a tripod.
The height of the mercury in the cistern would have been adjusted so that it met the tip of a small zeroing pointer on the cistern. At this point, the air pressure would balance with the weight of the mercury such that it would form a column in the tube of a particular height. A measurement could then be taken on the scale of the height of the column. For a more precise measurement of values between those of the main scale, the sliding vernier could be used. This instrument uses a ring-index vernier, which one would slide down until no light (reflected off a strip of paper) can be seen through the tube. This was meant to correct for any error in measurement from the meniscus.
The presence of the thermometer was also important for calibrating the instrument to temperatures other than that of the standard, and most barometers of the time included a thermometer.
From its provenance, this barometer may have been used in a chemical laboratory.
Historical AttributesUsed at the Wolcott Gibbs Memorial Laboratory (construction completed in 1913, torn down in the Summer of 2000).
Related WorksOn Fortin barometers: Middleton, W. E. Knowles. The History of the Barometer. Johns Hoptins Press, Baltimore: 1964, pg. 238, 350, 211-13, 346-48, 352-53, 371
On Henry James Green: On James Green: Middleton, W. E. Knowles. The History of the Barometer. Johns Hoptins Press, Baltimore: 1964, pg. 343-50