Mr. W. Brown on the Oscillations of the Barometer, 24-5 



3. It is not meant however in the foregoing paragraph that 

 the greatest depression of the barometer throughout the course 



with regard to temperature and the quantity of aqueous vapour they con- 

 tain; and hence the true nature of* the connexion between the occur- 

 rence of rain and a falling barometer, both consequences of one common 

 cause. 



The phaenomena attending the fall of rain are extremely dependent on 

 geographical position, and are by no means sufficiently known to enable us 

 fully to carry out these principles to the explanation of them, although I 

 am firmly persuaded that when better known we shall be able to do so. I 

 will however just refer to a few cases similar to that mentioned, of which 

 they do give a sufficient explanation, and which are notorious weather 

 laws : — the occurrence of rain ; — just before a change of the wind, or at 

 the time of the change, whether it be from north to south, or from south 

 to north, though the most conspicuous in the former case ; during a north- 

 east wind with a falling barometer (§ 5), and with a south or south-east 

 wind (occasioned by the junction of a north-east and south wind, see § 11). 

 The last of these is the most conspicuous in the portions of storms to which 

 § 16 refers, which in those parts where the wind is from south to south-east 

 are always accompanied by abundance of rain. It may be thought that our 

 dry winds from north-west (also formed by a north and south wind (§ 11)) 

 are an exception to these results; but it is by no means necessary that rain 

 should always occur at the meeting of these currents, for if the lower cur- 

 rent greatly predominates in dryness or in quantity, then it is evident that 

 there need be no precipitation of vapour in the form of rain. But there is 

 another reason why these winds should be in general free from rain. The 

 occurrence of rain in showers with squalls of wind, when the other portions 

 of the day are fine, is a case to which the principle before us strikingly ap- 

 plies, for these squalls almost always blow in a direction somewhat different 

 from that of the wind in the intervals between their occurrence ; thus 

 showing that they arise from an immediate onset of one or other of the op- 

 posite currents : now it is very easy to conceive that two bodies of air may 

 meet so as to produce rain, although their relative temperatures and quan- 

 tities of vapour may be so adjusted, that the resulting temperature is suffi- 

 cient to maintain the same quantity of vapour; for if the collision be sud- 

 den, by the law of the diffusion of gases and vapours the vapour of the 

 warm air will rush at once into the cold air, not waiting for the mixture to 

 take place; and hence, being subjected to its temperature, it is immediately 

 condensed and the rain is produced. Now (§ 11) the north-west wind is 

 one of the most constant winds, hence one of the most favourably disposed 

 for the gradual mixture of the opposite currents. This also explains the 

 occurrence of fine weather with a steady barometer, for stability in the 

 pressure of the atmosphere can only be produced by the stability of the 

 currents. The formation and disappearance of clouds without rain may be 

 explained in the same manner, — the precipitated vapour not being suffi- 

 ciently dense to form rain is again aerified when the cold air acquires the 

 temperature of the mixture. [For a full description of the differences and 

 relations of the distinct atmospheres of air and vapour by which the globe 

 is surrounded, and on which this reasoning is based, I need scarcely refer 

 the reader to the * Meteorological Essays' of the late Professor Daniel!, 

 where they are set forth with great perspicuity and precision.] 



But perhaps the fact most remarkably in accordance with this application 

 of the principle set forth in this essay is that general one, established by 

 W. Snow Harris by induction from a great number of particular instances, 

 that thunder-storms result from the collision of opposite currents ; for the 



