336 SCIENTIFIC RECORD FOR 1884. 



January , and somewhat less so in June antl February. Sometimes they 

 continue only for a few hours, occasionally even only one occurs, but 

 frequently they follow each other in an uninterrupted series for many 

 days. Occasionally they attain an amplitude of many millimeters; they 

 occur both with rising and fallinjy pressure, both during a maximum and 

 minimum, but most frequently during the latter, and least frequently 

 during the maximum. They have an undeniable connection with pre- 

 cipitation, so that whenever they occur there is, in most cases, shnultane- 

 ously either rain, snow, or fog, or the formation of clouds, but inversely 

 they are not an invariable accompaniment of such phenomena; they do 

 not appear to depend on the direction or strength of the wind. They 

 have a most interesting and undeniable connection with thunder-storms, 

 being, with very few c'xceptions, invariably to be observed, so that one 

 may determine the time and duration of a thunder-storm by these irregu- 

 larities of pressure. In order to ascertain how general or local these 

 perturbations are, Schonrock compares the records at Pavlosk and 

 St. Petersburg and finds that they occur together at both places, but 

 the character of the records made by the Wild barograph, namely, one 

 record every ten minutes, conceals very many of the details and renders 

 the comparison very unsatisfactory. {Z. 0. G. M., xix, p. 396.) 



270. [Schonrock has here initiated a study which must have long 

 claimed especial interest on the part of every one who has had access 

 to the records of a self-registering barometer. Of the barographs at 

 present in existence, one of the most sensitive instruments and one well 

 adapted to this class of investigations is that invented and constructed 

 by Hough and established at Albany, at the Dudley Observatory, the 

 hourly readings of which have been (]uite fully published and discussed 

 by him. A duplicate of this instrument was set. up at the Army Signal 

 Office in Washington in 1871, and still continues to perform satisfactorily. 

 Several others have been constructed introducing modifications of 

 Hough's construction, so that abundant opportunity has been offered 

 during the past fifteen years for studying, on a continuous and very 

 perspicuous register, the occurrence of these irregular and rather mys- 

 terious changes of pressure. We have been accustomed to consider that 

 these may be produced by either one or all of ti'C following causes: 



1. Sudden gusts of wind blowing across chimney tops ])roduce drafts 

 and lower pressures, or blowing into the window of an otherwise closed 

 room produce slight increase of j)ressures within the room where the 

 barometer is i)laced. 



2. The downfall of rain, cooling and dragging with it air, pushes 

 out from the region of rainfall as a gust of wind; this gust, as it pushes 

 against surrounding resisting masses of air, is compressed, whence a 

 temporary increase of pressure results in the locality preceding the 

 advancing rainfall, precisely as has been explained by Espy, Henry, 

 Koppen, and others. 



3. Every ascending mass of air must be accompanied by the descent 



