« AnteriorContinuar »
Occurrence of rain during the different times of day, recapitulated from
table No. 6 e and 6 f.
h. m. hm. h. m. h. m. h. m.lh. m. h. m. h. m. h. m. lh. m.lh. mlh. m.
h. m. 0 321 51 421 542 494 506 3717 276 4815 27/1 17 43 31
Observations on the motion of strata of clouds of different heights at Colonia Tovar.
Cir. and very high clouds. 2
Mid.stra. 7,500--9,000 h.
Middle. 2 Lower.
Middle. 1 Lower.
24 .... 3 s.
1 E.SE., fog
High. 21 Middle, 21 Lower.
Middle, 21 Lower.
Middle. 3 Lower.
Middle. 1 Lower. 2 High.
E. E.SE. SE. sse. s. s.sw.sw.w.sw w. W.NW NW.N.nw n. N.NE NE. ENE.
From this we see that during the time from April 3 to April 22–
The E and E.SE. currents have been chiefly in no other but the lower strata.
The N. cu rent has been in all three strata, the upper, middle, and lower strata.
In the upper region occur chiefly W., NW, W.sw., N.
COLONIA TOVAR, VENEZUELA, June 11, 1857. DEAR SIR: Your kind letter of March 5, was received by me in due time, and a little box with eight pounds of mercury, for which I thank you very much, came to hand somewhat later, on the 3d of May.
Soon after the receipt of the mercury I went to work to fill the barometer tube according to your directions; but with every new trial I found that the mercury fell more and more below its standard height, although I was certain there could be no air above it. At first I could assign no cause for this failure; but the fact that I was losing regularly at every new trial suggested to me the idea, that in handling the mercury the latter might have taken moisture from the atmosphere. Accordingly, I placed the barometer tube containing the mercury and a Torricellian vacuum in a nearly horizontal position cautiously over a brisk charcoal fire, and in this way heated the mercury for some time, until no more bubbles were disengaged. I was hereby especially struck with the great quantity of escaping moisture, and never thought that mercury could have taken up so much from the atmosphere during the short period required for filling the tube. Can this property be due to the nitric acid, with which the mercury may have been purified, and which is known to absorb moisture from the air ? Thus, by boiling, and at the same time making use of your directions, I succeeded perfectly well in bringing the level of the mercury up to its standard value. In such a damp atmosphere as this the boiling of the mercury seems to be indispensable.
I have now the pleasure to say, that since the 9th of May the barometer may be considered to be as correct and precise as when I first received it.
In a separate envelope accompanying my present communication, I have the pleasure of sending you besides the meteorological registers for six months, a short essay on the cause of the daily periodical variations of the barometer, and a number of tables and diagrams.
Tables No. 2 a and 2 b contain half-hourly observations on the daily periodical variation of the barometer for 31 days, made with a view to determine the precise time of maximums and minimums and the amount of daily amplitude. With regard to the latter, if we take the mean of every six days in succession, beginning with the 10th of May, we get the following mean amplitudes: .060, .070, .068, .066, .064; showing a gradual rise and fall in the numbers. The greatest mean amplitude is for the period from 16th to 21st May, so that even these additional numbers are still in accordance with the view taken with regard to the amount of amplitude for the different periods of the year, alluded to in my last letter. All the half-hourly observations up to June 9, inclusive, hitherto made by me on the subject of periodical variation, which are for 56 days, prove for the occurrence of the a. m. maximum, the average time to be at 10h. 104m. a. m., and for the p. m. minimum 4h. 31 m. p. m., which seems to agree pretty well with the time of daily inaximums and minimums found in other parts of the globe.
Table No. 3 is to exhibit the number of hours of rain during the different times of the day for each month from July, 1856, to May, 1857, recapitulated from tables No. 6 e and f. The vertical distance of the curve a b c from the base a d gives us the mean value of duration of rain for any given time of the day between 6 a. m. and 6 p. m. This curve is the expression of the mean for eight months from July, 1856, to February, 1857, and is laid down according to the mean numbers directly above it. It demonstrates very plainly that in the morning between 6 and 7 there was no rain; but with the advance of the day the rain augmented and reached its maximum between 2 and 3 p. m., whence it gradually abated towards evening. During the night it very seldom rains. Mr. Boussingault's observations, which he made in another part of South America near Marmato, prove that at that place more rain fell at night than during the day; and he says, in his Rural Economy, “every one in South America allows that it rains principally during the night.” Now this is in direct opposition with my observations here, and it shows, therefore, that a certain state of the weather, especially with regard to rain, may sometimes be limited to small districts only.
From table No. 3 we also see that the month of February, which is commonly considered to be one of the dryest of the year, and properly belonging to the very centre of the dry season, has been the wettest month of the year, with the exception of May. The dryest months were March and April.
By a glance at the tables No. 6 a to 6 f, we may have a ready survey over the dry and wet months of the year and the distribution of rain in general. Here we find that the limits of the dry and the rainy seasons are not very distinct, and from May, 1854, till the end of 1855, a period of 20 months, we find no well defined dry season, the month of February, 1855, being the dryest. But with New Year's Day, 1856, there commences a dry season which lasts for five months, the longest and dryest the colonists ever enjoyed. And it was in this extraordinary dry period that the loose layer of half decomposed vege