134 EXCURSIONS IN MADEIRA 



surgeon of a ship at Whydah, during the Harmattan, without a 

 single individual taking it, although they all sickened of it when 

 the inoculation was repeated on the cessation of this wind. The 

 Kamsin is called the " hot wind" in Egypt, and in Madeira (where 

 it is called the Sirocco by the British, and Leste by the Portu- 

 guese) it sometimes raises the thermometer to 90° in the shade; but 

 on the Gold Coast, if I recollect right, it lowers the thermometer 

 from 5 to 10 degrees. The easterly current from Cape Palmas is 

 always reversed during the Harmattan, and I have known a vessel 

 run up from Cape Coast to Sierra Leone in five days, by taking 

 advantage of this circumstance; it generally takes from three 

 weeks to a month to beat up there. I feel impatient for the 

 opportunity of making some hygrometrical observations during 

 the Harmattan, and propose to ascertain the positive quantity of 

 humidity contained in the air at that time, by means of a doubly- 

 graduated tube and a trough of mercury, allowing a small quantity 

 of air to enter at the tube, after the mercury within it has been 

 gradually raised to ebullition, noticing the quantity of air by the 

 great scale of the tube, and the height of the mercury by the lesser, 

 observing the barometer and thermometer, calculating the volume 

 of air contained ; afterwards introducing a drop of water to 

 saturate it, calculating its volume at the temperature of the 

 atmosphere, and with it, that of the air perfectly dry, deducting it 

 from the volume found in the first instance, and calculating the 

 weight of moisture contained in the residue. This strikes me to 

 be the surest method, when the occasion is too interesting to 

 depend on the mere comparative indication of hygrometers. 



The insufficiency of my means would have entirely deprived me 

 of instruments for the more interesting magnetic observations", 



■" La physique y pourra enfin obtenir aussi les lois de la distribution du magnetisme 

 terrestre; dont, partout I'interieur de I'Afrique, on n'a pas la moindre notion ; elle y 

 trouvera encore des donnees meteorologiques d'un inter^t extreme. — Biot (Review of 

 the Mission to Ashantee), Journal des Savans, Aout. 



