714 PHILOSOPHICAL TRANSACTIONS. [aNNO 1780. 



the lakes and pools, which had been formed by the heavy rains and the over- 

 flowing of the river, and producing, in such as partake of sea-water, a line sea 

 salt in large crystals, not unlike fossil salt. In the months of February, March, 

 April, May, and June, the wind blows almost constantly from between north 

 and west, called sea-breeze, except now and then a day or two it will be east, 

 which when it happens in April makes it excessively hot, the sun being then in 

 and about the zenith of Senegal, heating the vast plains of sand over which this 

 wind is to pass before its arrival there, which, reverberating the received heat, 

 may contribute to increase it; for Dr. S. had observed, that the same month in 

 the river Gambia was not hotter than any other wind, owing in all appearance 

 to the difference of the soil of the country, which is not sandy like that of 

 Senegal. Dr. S. thinks it is the dust of the sand raised by this wind which 

 makes the atmosphere look hazy. He saw, in the year 1775, in the month of 

 April, in a morning preceded by an easterly wind, such a dust imitating a fog in 

 the air, that he could not see above 10 yards. 



The weather grew calm, and about 1 1 o'clock in the ibrenoon the atmosphere 

 became clear, by depositing a brownish impalpable dust, which covered every 

 thing near a line in thickness. The same thing Dr. S. observed at sea from on 

 board a vessel in the month of March 1775, at the distance of about 5 or 6 

 leagues from the land near the latitude of Senegal. The wind having blown east 

 in the night, he found in the morning the sails, shrouds, and deck, covered with 

 an impalpable dust. The description given by Dr. Lind of the Harmattans of 

 the Coast of Guinea, seems to agree with the east wind at Senegal in almost 

 every respect, except that the damp vapour in the former is not perceptible in 

 this, for It dries every thing that will admit of it. Water poured on the floor 

 of a room, for the purpose of cooling the air, is dried up in an instant, and 

 there is some effect on the thermometer placed in such a room. Salt, sugar, 

 and the like substances, which are half melted by the damp air during the rainy 

 season, dry again in a few days into hard lumps. Such household hirniture as is 

 made of wood, thougli it has been ever so well seasoned, shrinks and loosens 

 where joined, or splits and cracks where glued. It dries and parches the skin of 

 the white people as well as the blacks, and makes it sqmetimes as rough as any 

 clear frosty weather in Europe would. The sky is commonly clear and without 

 clouds; but the atmosphere is hazy, which, in his opinion, as already observed, 

 is occasioned by the dust, perhaps in conjunction with vapours arising from the 

 surface of the earth and waters. These vapours, though not to be seen in the 

 open air. Dr. S. has perceived by their shadow on white walls, arising from pools 

 which were close to them ; but the air being so dry they are absorbed by it, and 

 no more perceptible as vapour. That the evaporation must be very great when 

 this wind blows, the method the blacks have of cooling water will evince. They 



