386 SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 



fiud all are included in the preceding except the one case of a current 

 freely rising in a quiet atmosphere, and which, cooling by expansion, 

 precipitates its aqueous vapor. To this class especially belong many 

 of our summer rains, but especially the tropical rains. 



But even our summer rains also behave, with respect to atmospheric 

 pressure, precisely as we have above described ; and with respect to 

 the tropical rains, we read in general that they exert scarcely any influ- 

 ence upon the barometer, not even disturbing the regular diurnal oscil- 

 lation. But upon this point only a computation of the observations 

 which are the numerical presentation of the influence of rain upon the 

 barometer can decide. Observations in our climate are not convenient 

 to sucb a computation, since we cannot properly eliminate the precipi- 

 tation due to the inflow of cold currents. Hence, I have chosen Batavia, 

 a place in the tropics near the equator, for which I have before me the 

 three years of hourly observations of pressure and of rainfall published 

 by Bergsma.'' 



Before I proceed to communicate the results of my computations, I 

 must state that I cannot participate in the view expressed by Eeye 

 (vol. 8, p. 180), who there says: "It can, indeed, occur that upon 

 mountain-tops, where the vapor condenses to clouds, the pressure may 

 also diminish; but, for all that, the barometer need not fall at the 

 place, 80 miles away, where the clouds discharge their rain." The 

 clouds are not to be considered as magazines, in which the previously 

 formed rain is carried along to be discharged anywhere. There where 

 it rains is the location of an increased precipitation, and the j>ressure 

 must sink deeper there than where clouds onlj^ form without developing 

 into rain. 



The hourly observations at Batavia show at once, even by a cursory 

 review, that in fact, as was expected, even the heaviest rains scarcely 

 disturbed the diurnal change iu pressure, and this stands forth with 

 greatest clearness in the averages. I took for computing the corre- 

 sponding pressures only the rainfalls which gave at least 10 millimeters 

 iu an hour. The pressures for one and two hours preceding and follow- 

 ing the rainfall (often of many hours' duration), as "well as the pressures 

 during the first, second, and third hours of rain, were extracted, were 

 separately reduced to the daily means by means of the mean diurnal 

 variation, and then united in a general average. I thus obtained the 

 following numbers, to which I have also added the mean rainfall for 

 each hour, so that one can perceive how intense these rains are. For 

 comparison, I add that in Vienna, on the average, an entire rainy day 

 in summer affords 5.0 millimeters of water, but in winter 2.7. The 

 mean of six of the heaviest rains during one hour from 1853 to 1871 is 

 20,5 millimeters. 



