492 Mr. W. Whitehouse on a New Instrument for [May 25, 



y^Vo of an ineh of mercury and of brief duration, and yet moving with 

 sufficient force to reduce to insignificance the inevitable friction of the pen 

 and working parts. 



The chief difficulties met were : — 



1st. To retain perfect sensibility to minute variations of pressure with- 

 out being constantly thrown out of range by the greater changes. 



2nd. To record on a very open scale, and yet not to exceed the usual con- 

 venient limits of paper and space. 



It was found necessary to abandon the idea of recording absolute 

 barometric measurements on such a scale, and to deal with minute differ- 

 ences only (hence the name "Differential microbarograph "), making the 

 instrument self-adjusting, so as to act differentially only, allowing brief 

 wave-like motions or pulsations of pressure to record themselves as such, 

 while a steady rise or fall of pressure should record itself by a line or trace, 

 whose mean distance above or below the base-line will indicate pretty 

 accurately the rate per hour of such rise or fall. 



This power of self-adjustment has been obtained by use of a capillary 

 tube communicating with the air-chamber and with the atmosphere, 

 whereby the equilibrium disturbed by changes of pressure is being con- 

 stantly restored, and the pen brought back to the zero. 



It is as if the ordinary barographic curve of pressure were made to serve 

 as the base-line, and these minute variations were made to record them- 

 selves above or below it, as though they were ripples or waves upon its 

 surface. 



The two hydraulic chambers connected by a siphon being in a state of 

 equilibrium, and the closed one being in pneumatic connexion with the 

 buried air-chamber, it is obvious that any change in the atmospheric 

 pressure exerted upon the water in the open one would disturb the equili- 

 brium and alter the levels, by causing water to pass from one to the 

 other. 



The end of .the siphon, however, opens into a small cylinder closed at the 

 bottom, and suspended in the water in the open chamber ; this responds 

 to the movement, and measures the flow of the water taking place between 

 the chambers, the degree of its immersion depending upon the quantity of 

 water it contains, and altering by its displacement the level of the water in 

 which it floats. 



Inverted in the water, and attached by a couple of silk lines passing 

 over pulleys to this cylinder, partly as a counterpoise, but also to cooperate 

 with it in producing the movements of the instrument, is another cylinder 

 of equal area, capacity, and weight, closed, at top, where it is subjected 

 to the pressure of the atmosphere, open at bottom, where its interior is 

 removed from such pressure by being suspended over the mouth of a tube 

 in pneumatic connexion with the buried air-chamber. 



These two cylinders, carefully balanced and suspended half immersed in 

 the water in the open vessel, are equally but oppositely acted upon by any 



