24 



Mr. J. A. Broun on Simultaneous [^^^y ^ 1^ 



IV. On Simultaneous Variations of the Barometer in India/' 

 By J. A. Broun, F.R.S. Received March 16, 1876. 



[Plate 1.] 



In this, the first of a proposed series of communications on the baro- 

 metric variations, it may be useful to recall the views held by men of 

 science as to the causes which produce them. 



Since Pascal showed, by the experiments on the Puy de Dome, that the 

 height of the mercury in the tube was lower the higher the station, it 

 was a natural conclusion when the column of mercury fell while the 

 barometer remained in the same place, that this was also due to a dimi- 

 nution of the mass of air above it. 



In order to satisfy the facts, afterwards discovered, connected with 

 diurnal and annual variations of atmospheric pressure, hypotheses were 

 proposed as to the modes in which the quantity of gravitating matter 

 pressing on the barometer was increased or diminished. The actions of 

 currents containing colder and denser or warmer and rarer layers of 

 air, the accumulation of the air thus conveyed over a station, and the 

 overflowing from one station to another were the most obvious methods 

 of explaining the variations of mass. The fact that cold air enters along 

 the floor of a room while the air heated by the fire ascends the chimney, 

 was a suggestion applied on a large scale to the whole globe. The polar 

 regions took the place of the door as the source of cold currents, and the 

 tropical regions represented the fireplace ; from this last the air ascended 

 the great terrestrial chimney, passed over more northerly and southerly 

 countries till it descended near the poles, to seek its way back to the 

 place whence it came. This hypothesis is represented by engravings in 

 many works treating of these variations, and the most ingenious figures 

 are made to cover the earth's surface, showing how the aerial movements 

 ought to be performed : ought to he, for there is a great want of the 

 facts which should show that the currents really move as they have been 

 represented. 



It was found, however, that, even with the aid of these h^^pothetical 

 movements, one of the most marked and most regular variations of 

 atmospheric pressure could not be explained. The barometer rises till 

 about 9 or 10 o'clock in the forenoon and evening, and descends till 

 nearly 3 or 4 o'clock, morning and afternoon. The amounts of these rises 

 and falls are themselves subjected to laws varying with many conditions 

 which no system of ascensional currents can satisfy. The pressure of the 

 vapour in the air was the only remaining source of variation depending 

 on mass which presented itself ; and a well-conceived hypothesis founded 

 on this element in connexion with aerial currents was supposed to 

 explain the whole phenomena. 



When we seek for evidence that the causes proposed are either true or 



