new york academy of sciences 585 



Summary of Papers. 



Dr. Mitchell gave an interesting account of the recent dis- 

 covery of a sixth and also a seventh satellite of Jupiter by 

 Professor C. D. Perrine at the Lick Observatory, and described 

 the details of the photographic method by which these satel- 

 lites were discovered last December and January. 



Dr. Mitchell also spoke of the discoveries of satellites of the 

 other planets, including the ninth satellite of Saturn, which was 

 found by Professor W. H. Pickering in August, 1899. 



Dr. von Nardroff defined the Piezic barometer as an instru- 

 ment to measure the atmospheric pressure by measuring the 

 elasticity of a portion of air. In the small pocket form 

 of the instrument exhibited, a piece of heavy barometer 

 tubing, of 3 mm. bore and about 12 cm. long, was provided 

 at its lower end with a pear-shaped bulb, having an inter- 

 nal volume equivalent to about 70 cm. length of the tube. 

 At its upper end the tube was provided with a second small 

 bulb containing about 1 c.c. of mercury. Entering into the 

 tube from above was a short tube having at its lower 

 end a capillary opening. Through this tube the mercury was 

 introduced. 



In using the instrument all the mercury is brought into the 

 upper bulb by inverting. The instrument is then turned into 

 the erect postion, when the mercury enters the main tube a few 

 centimeters, the exact distance depending upon the atmospheric 

 pressure. The less the pressure, and hence the less the elasticity 

 of the air, the more the mercury will enter. The mercury 

 stands in the upper portion of the tube and partly in the upper 

 bulb, without any tendency to run down the sides of the tube. 

 A scale on the main tube drawn by comparison with a standard 

 barometer indicates the pressure. 



To understand the theory of the instrument assume the 

 lower bulb replaced by a continuation of the barometer tubing 

 of equal volume. Let b stand for the standard barometer 

 height, m for the length of the thread of mercury entering the 

 tube, and a for the length of the column of compressed air. 

 Then from Boyle's law (pv = p'v') we have 



