236 M. C. Martins on the Nature and Origin of 



that it did not act on the hygrometer as a humid fog. Van 

 Swinden is not less explicit. At Franecker, in Holland, the 

 air was in no degree moist, and the hygrometers indicated 

 the maximum of dryness on the 23d June, a day on which the 

 fog was very dense. During the whole of this month the 

 weather was very dry. 



At Manheim, observers satisfied themselves that this fog 

 was not moist but dry, judging by the hygrometer, the 

 evaporation of fluids, the drying of moistened bodies, such 

 as hay and the dust of roads, and its continuance during rain. 

 At Padua, Toaldo finds it completely different from ordinary 

 fog, and notifies that the hygrometers indicated dry. At 

 Salon, in Provence, Lamanon observed that salts did not 

 deliquesce, and it did not cause the hygrometer to ascend. 

 In 1783, the salt pits of Hyeres crystallized fifteen days 

 sooner than usual. 



At Narbonne, however, after having been dry, this fog be- 

 came humid, owing to winds from the east, which prevailed 

 on the 26th, 27th, and 28th of June. " At Laon," says Cotte, 

 i: it began on the 18th June ; it was very low and as thick 

 as in December, accompanied by a very cold south wind. On 

 the 19th there was a considerable storm ; the fog appeared 

 afterwards to increase, and continued to be cold while the 

 south wind blew, that is, to the 24th. During this time, the 

 fog was very humid, as my hygrometers indicated to me. 

 On the 24th the wind changed to north, the air became warm, 

 and the fog altered its character ; it became dry, and might 

 be compared to a smoke rather than a fog. The heat and 

 dryness always increased, north and north-east winds con- 

 tinuing to prevail. 



A single observer, Maret, affirms that at Dijon this fog 

 appeared to him in every respect like ordinary fogs. He 

 perceived, however, that vegetables were dried during the 

 day. 



This evidence does not appear to me to invalidate that of 

 all the others, particularly when such observers as Van 

 Swinden, Toaldo, Senebier, and Lamanon, ascertained the 

 dryness of the fog experimentally. 



Density of the fo<j in 1783. — At Copenhagen, the sun was 



