206 CLOSES or 



enabled me to take the horary angles. The chronometer, at 

 the little island of Mucara, gave Ion. 78 13' 54". We passed 

 on the southern extremity of the Placer de San Bernardo. 

 The waters were milky, although a sounding of twenty-five 

 fathoms did not indicate the bottom; the cooling of the 

 water was not felt, doubtless owing to the rapidity of the 

 current. Above the archipelago of Saint Bernard and Cape 

 Boqueron, we saw in the distance the mountains of Tigua. 

 The stormy weather, and the difficulty of going up against 

 the wind, induced the captain of our frail vessel to seek 

 shelter in the Bio Sinu, or rather, near the Punta del 

 Zapote, situated on the eastern bank of the Ensenada de Cis- 

 pata, into which flows the river Sinu or the Zenu of the early 

 Conquistadores. It rained with violence, and I availed 

 myself of that occasion to measure the temperature of the 

 rain-water : it was 26'3, while the thermometer in the air 

 kept up, in a place where the bulb was not wet, at 24'8. 

 This result differed much from that we had obtained at 

 Cumana, where the rain-water was often a degree colder 

 than the air.* 



* As, within the tropics, it takes but little time to collect some inches 

 of water in a vase having a wide opening, and narrowing towards the 

 bottom, I do not think there can be any error in the observation, when 

 the heat of the rain-water differs from that of the air. If the heat of the 

 rain-water be less than that of the air, it may be presumed that only a 

 \iart of the total effect is observed. I often found, at Mexico, at the end 

 of June, the rain at 19'2 or 19'4, when the air was at 17'8 and 18. 

 In general, it appeared to me, that within the torrid zone, either at the 

 level of the sea, or on table-lands from 1200 to 1500 toises high, there is 

 no rain but that during storms, which falls in large drops very distant from 

 each other, and is sensibly colder than the air. These drops bring with 

 them, no doubt, the low temperature of the high regions. In the rain 

 which I found hotter than the air, two causes may act simultaneously. 

 Great clouds heat, by the absorption of the rays of the sun which strike 

 their surface; and the drops of water in falling cause an evaporation, and 

 produce cold in the air. The temperature of rain-water, to which I 

 devoted much attention during my travels, has become a more important 

 problem since M. Boisgiraud, Professor of Experimental Philosophy at 

 Poitiers, has proved, that in Europe rain is generally sufficiently cold, 

 relatively to the air, to cause precipitation of vapour at the surface of 

 every drop. From this fact he traces the cause of the unequal quantity 

 of rain collected at different heights. When we recollect that one degree 

 only of cooling precipitates more water in the hot climate of the tropics, 

 than by a temperature of 10 to 13 3 , we may cease to be surprised at thf 



