B. TERRESTRIAL PHYSICS AND METEOROLOGY. 83 



storms co-exist with merely local ones. The latter are, in- 

 deed, generally sporadic, and especially frequent in favorable 

 localities, such as mountains and forests. A passing cyclonic 

 storm is always favorable to the formation of local thunder- 

 storms. The peculiarly favorable conditions that prevail at 

 certain localities are shown in a very interesting way in the 

 work ofPrettner on the climate of Carinthia. Vierteljahres- 

 Revue der Natunoissenschaften, II., n., 190. 



A NEW BAROMETER OF LARGE SCALE. 



The great desirability of being able to observe the slight- 

 est changes in atmospheric pressure has led to the produc- 

 tion of many more or less unsatisfactory barometers, of 

 which in general it may be remarked that, although they do 

 really afford us a highly magnified scale of movement, yet 

 the moving parts are themselves so weighty that the sluggish 

 behavior of the instrument entirely neutralizes the advantage 

 which was sought, so that the slight momentary changes in 

 atmospheric pressure still pass by un perceived. To meet 

 these difficulties, Mr. Hirn, one of the most eminent French 

 philosophers, has described an instrument which he calls the 

 Megabarometer : his apparatus consists of three vertical glass 

 tubes, closed at their ends, and connecting with a horizontal 

 tube by means of iron sockets. The middle tube, filled with 

 mercury up to half an inch of its top, is a true barometer. Its 

 neighboring tube on the one side has about four millimeters' 

 internal diameter, while the other tube has one millimeter 

 diameter, but is soldered at its top to a closed bulb of about 

 four centimeters' internal diameter. The lower half of this 

 bulb is filled with mercury, the upper half with alcohol. 

 The first of the three tubes thus constitutes a barometer com- 

 posed of two liquids, and the variation of level in the two 

 open tubes on the right and the left hand is very nearly in 

 an inverse ratio to the densities of the liquids; so that a 

 change of one inch in the height of the mercury brings about 

 a change of seventeen inches in the alcohol tube. JSTouvelles 

 Meteorologiques, p. 34. 



MIRAGE. 



In some remarks on the phenomena of mirage, Professor 

 Everett states that when a ray of light is passed through a 



