Jökull - 01.01.2001, Page 79

Observations on the dry fog in June 1783
Table 1. Tabulation of van Swinden’s weather observations in late June 1783.
Date Thermometer

Barometer

Hygrometer

Visibility

Wind

Max. Med. Buiff. DeLuc
18 13.1 12.6 27,10,1 15.8 40.9 Thick air SW
19 16.4 14.2 27,9,9 16.7 42.4 Thick air SE,SW
20 16.2 14.0 27,10,0 13.6 31.3 Thick air E,SW,NW
21 14.9 13.6 28,0,6 16.5 41.8 Thick air NW
22 14.4 12.5 28,4,2 24.4 51.2 semi-clear NW
23 17.6 14.6 28,6,7 32.5 67.2 semi-clear NW
24 21.7 18.9 28,5,2 37.4 75.2 semi-clear E,NE,N
25 22.1 18.6 28,3,6 26.8 54.3 semi-clear NE
26 19.1 16.5 28,3,3 21.6 45.4 Clear N,NE
27 18.4 16.3 28,3,4 25.9 54.7 semi-clear NW
28 18.6 16.9 28,3,6 28.5 58.6 Thick air NW
29 19.1 17.4 28,4,3 27.8 57.6 Thick air NW
30 20.4 17.8 28,5,6 27.7 51.0 semi-clear NW

The temperature measurements are given in degrees Réaumur (=1.25

C); max. and med., indicate highest and average
daily temperature. In the original table it is also noted that the atmospheric conditions were cold (frigus) between 18-22
June and hot (calidium) between 23-30 June.

The barometric readings are given in inches. In the original table it is indicated that atmospheric pressure was low between
18-20 June, at medium elevations between 21-23 June, and high between 24-30 June.
Hygrometer readings are given in units pCt (see Traumüller, 1885). Swinden used two brands of hygrometers, Buiffardi
(Buiff.) and de Luc’s (DeLuc). The hygrometer measurements show a drastic increase at the time when the haze was of
greatest intensity; hence, higher readings indicate drier conditions. In the original table it is also noted that the atmosphere
was humid (humidus) between 18-20 June, very dry (siccissimum) between 23-25 June and dry (siccum) between 26-30
June.
Here van Swinden gives comments on the conditions of the sky. He also notes that a violent thunderstorm with heavy rain
occurred on the 20 June.
Here van Swinden gives the direction the wind was blowing from each day. He also notes that from the evening of 20
June to 22 June the wind was of medium strength. In the days that followed the wind was very weak.
first became orange, then purple (violet) when washed
with water. The black paint was partly washed away.
The purple lost its vivacity.”
Paintings, exposed to humidity by M. du Vasquier,
changed the most. He also noticed that when they
were kept up high and dry, no alteration occurred.
Moreover, he noticed with amazement, that on the sur-
face of a painting, half of a flower could fade whereas
the other half kept its gloss. It has to be noticed that,
whatever was the compound from which the painted
flowers were made, the effect was always the same.
Based on M. du Vasquier observations, he con-
cluded that the fog was some gas mixed in the air
without being dissolved.
He does not know the nature of this gas, but he
concludes from his experiments that it was not en-
tirely mixed with the air like butter in milk. It can
also be concluded that it is heavier than air because the
summit of the Alps could be seen but not the foothills.
The barometer was constantly high during the fog pe-
riod.
This gas was mixing with water to a certain point
because heavy rainfall could wash it away and it was
acting on color only through water. This gas was acid
JÖKULL No. 50 79