1890.] during Thunderstorms, and on the Brontometer. 61 



the middle of this rise, sudden heavy rain begins. After a few 

 minutes the rain, with or without thunder and wind, becomes a little 

 less heavy, and the barometer sometimes falls a little. The rain 

 then continues till the end of the squall, and as it stops the barometer 

 returns to its original level. In Great Britain the rise rarely exceeds 

 O'lO in., or lasts more than two hours. These rises are always 

 superadded to a more general rise or fall of the barometer, dne either 

 to a cyclone or to one of the small secondaries which are formed on 

 the side of one. During some rises the wind remains unchanged ; 

 with others there is a more or less complete rotation of the wind. In 

 all cases the disturbance seems to be confined to the lower strata cf 



the atmosphere." " Since the rise is always under the visible 



storm, it is propagated at the same rate and in the same manner as 

 the thunderstorm. Enough is known of the course of the latter for 

 it to be certain that they are not propagated like waves or ripples, 

 and hence these small barometric rises are not due to aerial waves, as 

 has been suggested. Since their general character is the same 

 whether there is thunder or not, it is evident that electricity, even of 

 that intensity which is discharged disruptively, is not the cause of 

 the rise. If we look at a squall from a distance, we always see 

 cumulus above it, which is harder or more intense in the front than 

 in the rear of the squall. Since cumulus is the condensed summit of 

 an ascensional column of air, it is evident that the barometric rise 

 takes place under an uptake of air. If we consider further that a 

 light ascensional current would give rise simply to an overcast sky, a 

 stronger one to rain, while a still more violent one would project the 

 air suddenly into a region so cold and dry that the resulting elec- 

 tricity would be discharged disrnptively as lightning, the foregoing 

 observations show that the greatest rise is under the greatest uptake. 

 Our knowledge of the mechanics of fluid motion is still too unsettled 

 for us to say with certainty whether or not an ascensional current of 

 air would have a reaction backwards, like a jet of air issuing from an 

 orifice." 



Professor Mascart also, in 1879, expressed the opinion that elec- 

 tricity had nothing to do with these oscillations, but suggested quite 

 a different explanation. Premising that they are not produced by all 

 heavy rains, but only when heavy showers fall during bright weather, 

 he suggested that at such times rain falls through non-saturated air, 

 where it would evaporate freely, and so produce a local increase of 

 pressure which in certain thunder-rains might amount to 2 mm. 

 (0'08 in.). He explained the diminution of pressure which some- 

 times occurs, by the reversed phenomenon ; he considered that 

 thunderstorms are formed locally, and suggested that the condensa- 

 tion of masses of vapour into rain drops ought to produce a diminu- 

 tion of pressure. 



