92 A. A. Michelson — Air-thermometer. 



former to be a little the greater. But if the principle of the 

 ^utility of the ah-orbing ami radiating powers of all bodies is 

 true, which is general!}' conceded, thus a greater absorbing 

 power, if it is accompanied by a proportionately greater radiat- 

 ing power, cannot give rise to a higher temperature, and to 

 account for any difference in the temperatures of the two hemis- 

 pheres, it would be necessary to suppose that the absorbing 

 and radiating powers are different, cither in a land-, or in a 

 water-surface, or in both. 



The establishment, therefore, of the equality of the moan 

 temperatures of the two hemispheres seems to confirm tue 

 principle of the equality of the absorbing and radiating powers 

 of b-dies. since this seems to be the case with regard to water, 

 and land with all its variety of surface. If this were not true 

 for any considerable part of the earth's surface, it would affect 

 the equality of the mean temperatures of the two hemispheres. 



era of the Barometric Pressure ; by Albert A. MiCHELSON. 

 The appearance of an abstract of a paper by Pettersson* on 



more manageable than that which is there described, and which 

 likewise retains the important advantage of giving indications 

 which are independent of the external pressure. 



The instrument consists of a -lass bulb and stem, the former 

 about 40 mm and the latter about 2" n ' ! in interior diameter. The 

 bulb contains dry air at a pressure of about 100 mm of mercury, 

 and this air is separated from the upper portion of the tube by 

 a column of mercury about 100 mm in length. The mercurv 

 remains above the air, notwithstanding the large diameter 01 

 the bore, owing to the resistance to deformation of the 

 menisco.8. The space above the mercury is a vacuum. 



Thus the pressure of the air in the bulb is constant and is 

 equal to that of the column of mercury above it. If the bore 

 of the stem is not of uniform section the length of the column 

 will change— but this length is easilv read oil" and gives at once 

 the true pressure. 



The pressure need not be limited to 100 mm , but if it be much 



The only precaution to be observed, bevoud what is used in 

 an ordinary mercury thermometer, is that the stem must be- 



