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component or foi'ce aftei-wards acting on the mass wonld be towards the earth's 

 centre, or vertically downwards, owing to the attraction of gravitation. Take 

 a straight vertical glass tube, open at the top and closed at the bottom, the 

 lower portion containing air, above which is a column of mercury. Choose a 

 tube with a bore not exceeding one-eighth of an inch in diameter, in order to 

 take advantage of the cohesive attraction of the mercury, which prevents its 

 separating into small drops and falling to the bottom of the tube ; if, now, the 

 tube be raised suddenly, then, from the inertia of the mercui-y, the column of 

 air is compressed, and if we keep the tube motionless, the compressed air will 

 react on the mercury, which will then indicate every change of pressure in the 

 column of the air. 



It is considered, in what is called the wave theory of the atmosphere, that 

 the point of greatest elevation, or crest of the air, corresponds to the highest 

 reading of the barometer, and that the lowest point, or trough, corresponds to 

 the least pressure. It must be borne in mind, however, that we are not 

 considering the case of an incompressible liquid, but that of a compressible gas, 

 varying from a state of extreme tenuity in its highest regions, to that state of 

 elasticity which it exhibits at the earth's surface. 



It follows, therefore, that if we start with an atmosphere free from all 

 vibration, and that for simplicity we suppose a single impulse to be given to a 

 column, the time between each oscillation would be uniform ; and from 

 examination of barometrical observations at this and other stations, it appears 

 that there is a very frequent oscillation of about three and a-half days, that is 

 from maximum to minimum pressure. We will neglect, for the present, the 

 small horary variations so well established, which are also observable at this 

 station, and take the barometrical tables for 1869, at Dunedin. The lowest 

 pressure for the month of January (reduced) was 29-263 inches on the 20th ; 

 the highest being 30-252 on 23rd ; while on the 26th it was down to 29-4:16. 

 In the month of February, the barometer was at 29-689 on the 5th, and the 

 highest reading for the month (and year) was 30-462 on the 8th ; while three 

 days subsequently, on the 11th, the pressure was 29-8. On the 20th, or 

 nearly three vibrations afterwards, it was as high as 30-34 ; while on the 24th 

 it was down to 29-417, which was the lowest for the month, and on the 27th 

 it stood at 29-8 inches. In the month of Mai-ch the pressure was 30-239 on 

 the 20th, and 29-130 on the 24th, while it rose to 30-147 on the 28th. In 

 April we have 29-89 on the 1st, and 30-430, the highest for the month (and 

 near the highest for the year), on the 5th. Then 29-978 on the 9th j 30-223 

 on the 11th; and four days latei*, 29-142, being the lowest for the month, 

 occurred on the 15th, while on the 18th it stood at 30-036. In May, 29-706 

 on the 16th, and 30-390, being the highest for the month, on the 20th, while 

 the lowest for the month, and also for the year, namely, 29-027, occuri-ed on the 

 24th, and on the 29th it rose to 30-031. In June, the highest reading for the 



