86 ' On Barometric Pressure. 



the atmosphere, a question arose respecting the forces imme- 

 diately under a balloon ; and more particularly, the pressure of 

 the atmosphere downwards. 



I venture to send to you a few questions, which have arisen 

 in my own mind, during a short consideration of the subject; 

 intended, if you think fit, for insertion in your Magazine. I re- 

 frain from giving you what I have heard on the suiyect, because 

 the conversation in which it occurred was private ; — and my 

 object at present is, to make a confession of my own ignorance 

 of this part of experimental philosophy, and request information, 

 through the medium of your Magazine, from those who are able 

 to give it. 



If a balloon in a state fit for ascension is liberated from the 

 earth's surface, T conceive the barometrical pressure beneath, is 

 diminished for a short time ; i.e. whilst the l)alloon remains in 

 the vicinity of the spot from whence it was lil)erated : because, 

 as the balloon, with the column of air above, is lighter than the 

 collateral corresponding columns ; it, whilst rising, must cause a 

 diminished downward pressure on the .spot beneath. It has been 

 said to me, that, the pressure of the air taking place in all di- 

 rections, that of the columns surrounding the balloon will be 

 exerted as well under it as laterally, and produce the same 

 effect there as on any other part. This I cannot conceive to 

 be the case, until the balloon is so far removed from the eartii's 

 surface, as not to influence the air on it ; or, in other words, until 

 the distance is such, that of all tiie air which flows in to supply 

 the place of the balloon as it rises, none comes from the earth, 

 or from that part surrounrling the barometer. But whilst the 

 air which replaces tbe balloon forms a current enveloping the 

 instrument, the mercurv in it I think would descend. 



I dare not positively affirm that I am right in these conjec- 

 tures, and therefore ask your assistance to obtain information 

 concerning them — but I will describe to you an experiment I 

 made on the subject. 



I bent about two inches of the end of a long open glass tube 

 upwards to an acute angle, and then immersed it to some depth 

 beneath the surface of water, in a large vessel ; inclining the 

 longer leg of the tube, so as to remove the point wiiere it touched 

 the surface of tlie water to some distance from the spot iinme- 

 diately over the lower opening. In this situation, the tube con- 

 tained a column of water which could not be affected by the 

 general mass in the vessel, except at the lower end. Then, I 

 was able by means of the hmgs, and a large tube, to liberate 

 considerable bubbles of air in the water at any part. Permitting 

 the bubbles to escape over the orifice of the tubc^ they caused 



a colli 



