1871.] On a new Atmospheric-Pressure Instrument. 491 



A fair measure was obtained of the centre of the middle band, which was 

 the brightest; it gives for this band a wave-length of about 510 millionths 

 of a millimetre. I was not able to do more than estimate roughly the 

 position of the less refrangible band. The result gives 545 millionths. 

 The third band was situated at about the same distance from the middle 

 band on the more refrangible side. 



It would appear that this comet is similar in constitution to the comets 

 which I examined in 1868*. 



V. te On a New Instrument for recording Minute Variations of At- 

 mospheric Pressure." By Wild man White house, E.M.S.&c. 

 &c. Communicated by R. II. Scott, F.R.S., Director of the 

 Meteorological Office. Received May 8, 1871. 



(Abstract.) 



The occurrence of a heavy " ground-swell " on the sea-coast in per- 

 fectly calm weather suggested to the writer some years ago the possibility 

 of atmospheric waves or pulsations accompanying a gale being propa- 

 gated to a considerable distance (irrespective of any horizontal movement 

 of air), and giving evidence of the disturbance existing elsewhere. 



It was seen that, even if such were the case, it would be difficult to 

 obtain proof of it, as any ordinary observations would fail to detect it, and 

 that it could only be attained by the adoption of a system of continuous 

 record specially adapted to the purpose. The writer therefore determined 

 to design and construct an instrument with this object ; and after many 

 trials with varied apparatus, it was decided to adopt the hydraulic prin- 

 ciple, as affording at once the means of accumulating force sufficient to 

 actuate the instrument, and of measuring the force itself by the alteration 

 produced in the height of the column of water. 



The use of an air-chamber was suggested by the sympiesometer ; it has, 

 however, been enlarged to meet the altered conditions of this instrument, 

 and is buried underground to secure freedom from all diurnal changes of 

 temperature. 



The action of the instrument essentially depends upon the flow and 

 reflow of water between two hydraulic chambers (connected by a tube or 

 siphon), one of which is open and exposed to atmospheric pressure, the 

 other closed at top, and removed from such pressure, being in pneumatic 

 connexion with the buried air-chamber. 



Any difference in the levels of the water in these two chambers is a 

 measure of the variation of pressure producing it, and the water in its flow 

 is made to move the tracing-point or pen across the paper. 



In order that the objects of the research should be attained, the action 

 must be continuous, unfailing, of great delicacy — able to show changes of 



* Phil. Trans. 1868, p. 555 ; and Proc. Eoy. Soc. vol. xvi. p. 386. 



