432 



Prof. F. Kolilrausch on the Determination of 



nishes a very delicate means of measuring pressure. From the 

 smallness of the mass put in motion in the action of this instru- 

 ment, the initial oscillations are of very short duration. The 

 moment of inertia of a manometer which is specially intended for 

 such experiments, may be materially diminished as compared with 

 that of the commercial instruments, in which little attention is 

 ordinarily paid to this element of delicacy. 



I will here communicate a few observations which I made at 

 the instigation of Professor Weber, which cannot indeed serve 

 for more than a preliminary trial of the method, and should in- 

 cite to a more accurate repetition with more perfect means. 



The instrument used was a Paris barometer graduated in milli- 

 metres. It was placed on the plate of an air-pump under a re- 

 ceiver of about 6 litres capacity. The air under the receiver was 

 dried by means of chloride of calcium. By a rapid stroke of the 

 piston, the air in the receiver was rarefied and immediately shut 

 off by a stopcock. The index of the barometer at first moved ra- 

 pidly towards the smaller numbers, and then retrograded, at first 

 rapidly, and then more slowly through a number of divisions. 

 During this time one observer gave a signal as often as he noted 

 that the index passed over a whole division ; a second noticed the 

 corresponding time. When the motion of the index had become 

 slower, parts of a division were noted. After a lapse of sixty 

 seconds a motion could no longer be perceived; that is, the mass 

 of air had assumed the temperature of the surrounding atmosphere. 



With the aid of Dr. Nippoldt the six following series of ex- 

 periments were made, from which a mean may be easily de- 

 duced. The diminution in pressure of the air, which before the 

 experiment was under atmospheric pressure, was nearly equal in 

 all experiments ; after the original temperature had been restored, 

 the maximum was 38*5 millims. and the minimum 34 millims. 

 As the individual series are proportional, they can all be reduced 

 to the mean alteration in pressure, 37 millims. The observations 

 thus corrected are contained in the following Table, in which t 

 represents the time in seconds which elapses from the beginning 

 of the stroke of the piston, y the distance in millimetres of the 

 index at the time t from its ultimate position. 



t. 



y> 



t. 



?/• 



t. 



y- 



t. 



y- 



t. 



y- 



t. 



y- 



seconds. 



millims. 



seconds. 



millims. 



seconds. 



millims. 



seconds. 



millims. 



seconds. 



millims. 



seconds. 



millims. 



20 



71 



2-0 



8-0 



20 



8-2 



21 



7-4 



20 



7-55 



2-3 



75 



40 



5-2 



3-8 



5-9 



3G 



6-2 



3-8 



5-5 



3-9 



5 65 



41 



5-5 



51 



4-2 



60 



37 



51 



4-3 



60 



3-6 



0-2 



375 



61 



3-6 



8-3 



2-2 



8-0 



2-6 



8-1 



23 



80 



2-6 









8-2 



2-6 



12 



1-3 



10-3 



1-5 



110 



1-4 



110 



1-6 



101 



175 



10-8 



1-6 



210 



03 



200 



0-4 



194 



0-4 



18-2 



07 



202 



0-35 



18-3 



0-6 



40 



01 



40 



0-2 



40 



005 



40 



0-1 



35 



015 



40 



9-1 



