SUPPLEMENTARY REPORT ON METEOROLOGY. 87 
137. Mr. Harris, of Plymouth, has had the merit of exe- 
cuting, for the first time, perhaps, with accuracy, a wheel, or 
circular index-barometer, a matter of very great importance as 
well as difficulty, because it enables unlearned persons to read 
off the height without the aid of a vernier; and in fact to this 
we owe the most valuable and unique series of hourly barome- 
trical observations, to which we shall presently refer*. 
138. Prof. Stevelly, of Belfast, has proposed a self-registering 
barometer, on the principle of causing a moveable cistern con- 
taining mercury to rise or fall by the weight of the fluid dis- 
placed from a fixed barometer tube immersed in it. The tube 
here may evidently be opaque, as of iron. 
139. A water-barometer has been constructed and observed 
in the Royal Society’s Apartments (London). Some elaborate 
comparisons of its indications with standard instruments will 
be found in Mr. Hudson’s paper f. 
140. On the still-agitated subject of capillarity, as affecting 
barometric readings, I refer to some recent essays of Bessel, 
Dulong, and Bohnenberger §. 
141. Of barometers acting on principles different from that of a 
simple column of mercury, the so-called (not very appropriately) 
differential barometer of Auguste, as improved by Kopp], is 
the most ingenious. Its principle, so far as it can be concisely 
stated without a figure, is this: if air of any density whatever 
be compressed into a given fraction, say $ths of its natural bulk, 
it will sustain a pressure equal to the atmospheric pressure, and 
a certain fraction more, depending on the fraction denoting the 
compression (in the supposed case its elasticity would be 
balanced by the atmospheric pressure, and ird more). If, then, 
this fraction of excess of pressure is known by experiment, the 
whole pressure is inferred from a knowledge of the construction 
of the instrument. Thus, instead of a column of 30 inches of 
mercury being required, one of 15, 10, or any other number 
may be used and multiplied by the constant factor. In Kopp’s 
instrument, the experiment is very simply and neatly made. A 
glass chamber communicates freely with a vertical tube, which 
is open to the air until mercury forced in from beneath cuts off 
the communication: the pressure by which the mercury is in- 
troduced being continued, the air in the chamber is condensed, 
and the mercury: rises in the vertical tube, so that its pressure, 
together with that of the atmosphere, may balance the elas- 
ticity of the air. The compression is continued until the air is 
* British Association, Third Report, p. 414. 
+ Transactions of the Royal Irish Academy, 1836. 
{ Phil. Trans. 1832. § Poggendorff, xxvi. 451. 
|| Poggendorff, xl. 62. 
