INTRODUCTION. x j 



to, with this difference, that the precipitation in its descent does not 

 become again absorbed. 



The experiments I have occasionally been able to make on land, 

 relative to the formation of dew, have been but few during the voy- 

 age, but I feel satisfied that the same phenomenon occurs without a 

 change taking place in the temperature of the soil, particularly in low 

 latitudes, and that the deposition is attributable to causes operating 

 in the atmosphere above, as before indicated; and fully believe that 

 these depositions are far more beneficial to vegetation, and conse- 

 quently to the agriculturist, from the fact that the soil does not 

 undergo any change as to temperature that would serve to check 

 growth, at the same time that it enjoys the beneficial effects of the 

 moisture. 



Between the latitudes of 5 and 9 30' north, we experienced the 

 equatorial rains : the amount which fell in ten days was 6'15 inches, 

 its temperature 69 to 72; the. greatest fall in twenty-four hours, 

 l - 95 inch. The variable or rainy zone, at the time we crossed it, ex- 

 tended from 4 to 12, north latitude, which gives it a width of five 

 hundred miles. The maximum heat of air and water occurred in 8 30' 

 north, and in 24 west longitude ; which corresponds very nearly to 

 the centre of the belt. We were detained within it twenty-three 

 days, during which time the mean standing of the barometer was 

 29-987 inches, and the mean temperature of the air 79'6, and that 

 of the water 82*07; the degree of moisture -734; the winds were 

 light and variable, generally from the northward and westward, 

 though inclining to calms; the movement of the upper stratum of 

 clouds was to the westward during our traverse across the zone. 



On the equator, the oscillations of the barometer were found to cor- 

 respond to 3 A. M. and 3 p. M., 9 A. M. and 9 p. >i. ; the variation was equal 

 to -1 of an inch; this is the result of hourly observations for a period 

 of forty-eight hours ; the actual observations have been already pub- 

 lished, and will be found in Appendix XVI., Vol. I., of the Narrative. 

 The southeast trades were entered in 3 50' north latitude: these 

 winds, at the season of the year we passed through them, were found 

 to increase as the sun was approaching the meridian, and to continue 

 until the afternoon, when they decreased until towards evening, after 

 which they again increased until towards daylight. The weather 

 from the latitude of 3 north is generally fine throughout the year. 

 A variation in the monsoons is experienced as the coast of Brazil 

 is approached : they blow from October to March from the northward 



