394 SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 



The objection tliat the tropical rains (and the summer rains) are of 

 much more local nature than the winter rains of temperate and arctic 

 latitudes I can only designate as a make-shift, for the great intensity of 

 the tropical (and summer) rains must make up for their slight extent 

 at least so far that a tendency to a fall of the barometer must be evi- 

 dent in the observations. The demonstration that the tropical rains 

 always have a less extent than the precipitations accompanying the 

 storms of higher latitudes must at any rate be first produced,* and after 

 that the proof will still have ro be given showing why the heat liberated 

 by the ten and twenty times heavier equatorial rains cannot cause an 

 ascending current intense enough to cause a depression of even 2 or 3 

 millimeters in the barometer at the earth's surface, while our winter 

 rains are accompanied by depressions of 30 or 40 millimeters. 



We have, on the other hand, no observations that would, beyond doubt, 

 prove that the formation of clouds and raiu is alone able to sensibly 



* I am persuaded that iu many tropical countries, especially in the equatorial zone 

 itself, during the rainy period, precipitation occurs simultaneously over a region which 

 is at best as large as that which corresponds to the last half of our own revolving 

 storms, lb is a rather widesjiread error that the rainfall in the tropics always occurs 

 in the form of local thunder-storms. There occur equally uniform and constant rains 

 there, as is the case with our own general rains. On this point, I refer only to the 

 Reports of Frautzius on the Atlantic Coast of Central America (" Zeitschrift fiir Erd- 

 kunde," 1888), of Humboldt and of Bates ("Der Naturforscher am Amazonenstrom ") 

 on the region of the Amazon ; e. g., Bates says : " From the 14th to 18th January the 

 weather was bad; it rained sometimes twelve hours in succession, not heavy, indeed, 

 but steadily drizzling, as in England." As soon as we approach the equator within 3° of 

 latitude, we seldom find opportunity to observe sun and stars. The missionary in San 

 Antonio de Javita assures us, says Humboldt, that he has often seen it rain here four 

 or five months without cessation. These are certainly no local rains, for these remarks 

 refer to the uniform lowland of the Middle Amazon and to the Rio Negro. Even in 

 Vienna it is seldom that rains continue twenty-four hours or more. According to the 

 five years of registers of a rain-autograph, there occur on the average in a year only 4.3 

 precipitations that last over twenty- four hours. (See Hanu, Met. Windrosen fiir 

 AVien, Sitzb. K. Akad., 1867.) 



By reason of the great attention that travelers have given to the regular daily changes 

 of the barometer, it would be highly remarkable if any connection between the rain 

 and the barometric depression had escaped them in a climate where there are no irreg- 

 ular changes of pressure in consequence of changes of wind-direction. And yet one 

 finds no no!e as to the presence of any such relation between rain and pressure — the 

 remarks express themselves directly against such connection. I will here quote only 

 one passage from Herschel's Meteorology, page 163: "Colonel Sykes remarks (Phil. 

 Trans., 1850) that, among many thousand observations taken personally by himself on 

 the plateau of the Deccan (1825-1830), there was not a solitary instance in which the 

 barometer was not higher at 9 or 10 a. m. than at sunrise, and lower at 4 or 5 p. m. than 

 at 9 or 10 a. m., whatever the state of the weather might be." Humboldt also observes 

 (tome 1, p. 308): "This regularity is such that, in the daytime especially, we may infer 

 the hour from the height of the column of mercury without being in error on an aver- 

 age more than 15 or 17 minutes. In the torrid zone of the new continent I have found 

 the regularity of this ebb and flow of the aerial ocean undisturbed either by storm, 

 tempest, raiu, or earthquake, both on the coasts and at elevations of nearly 13,C00 feet 

 above the level of the sea." 



