﻿432 Mr. J. K. Laughton on Barometric Differences 



it is asserted that, by reason of the comparative vacuum formed 

 high overhead, the air rushing in forms an ascending current, 

 the centre of depression becomes a species of chimney up which 

 the air is carried with great velocity, and that this ascensional 

 motion accounts for the full extent of the loss of pressure. But 

 any ascending current so formed is due directly to the conden- 

 sation, and must occur wherever condensation is excessive ; any 

 depression of the barometer caused by it is essentially a compo- 

 nent part of the depression caused by heavy rain, and cannot be 

 separated from it in any observations. Since, however, consider- 

 able stress has been laid on the influence of this ascensional 

 motion, it is worth while to inquire into how far it really exists, 

 and what effect it is really capable of producing on the barometer. 

 It has always been a favourite opinion amongst meteorologists 

 that the ventilation of coal-mines by means of furnaces offers a 

 convenient and apt illustration of the movements, and the causes 

 and effects of the movements, of the atmosphere. The correct- 

 ness of this opinion is, I think, more than doubtful : I have 

 already called attention to the fact that the differences of tempe- 

 rature produced in the furnace method of ventilation are, beyond 

 all legitimate comparison, greater than any that occur in nature; 

 and not only these excessive differences of temperature, but also 

 the friction of the currents of air driven at a high velocity 

 through narrow and winding passages, or what is technically 

 called " the drag of the air," produce a very marked effect on 

 the pressure as shown by the barometer or gauge, which must 

 be eliminated before we can tell how much of any registered 

 difference between the air of the upcast and downcast shafts is 

 due to the ascending or descending motion of the current. Cap- 

 tain Toynbee has recorded that, observing with an aneroid, he 

 "found a difference of a tenth of an inch between the pressures 

 at the bottom of the downcast and upcast shafts, both positions 

 being on the same level, whilst halfway down each shaft there 

 was only a difference of four hundredths of an inch "*. By the 

 kindness of Mr. Landale, of Lochgelly, I am enabled to present 

 the results of some similar observations which he has recently 

 had taken at my request. They are as follows : — 



Name of pit. 



Number of 



observations 



(double). 



At a depth in fathoms. 



Mean reading of 

 aneroid. 



Upcast. 



Downcast. 



Upcast. 



Downcast. 





6 



7 

 4 



70 

 70 

 24-5 



70 



715 



25-5 



30190 

 30-316 



29788 



30-215 



30-350 

 29-848 



Foulford 



Pit No. 21 .... 



Meteorology of the North Atlantic, p. 12. 



