39 
Mr. W. W. Stoddart, F.G.S., exhibited a small Aneroid barometer in 
his possession, which he had compared with 19 others, and found none 
more accurate, provided certain precautions, which he described, were 
observed in reading it. With several independent observations he had 
ascertained the actual height of a staircase by it within one foot. The 
advantage of the portability of this instrument was very great, as well as 
the saving of time and trouble in making an observation. He also 
explained the construction of the compensating spring, which obviated the 
necessity of a correction for temperature. 
Canon Moseley referred to Galileo as having first used the barometer 
for measuring heights, in the Puy-de-dome, 200 years ago. The subject 
of the paper was an example of the combination of mechanical skill with 
great scientific knowledge. He considered Mr. Sanders had greatly con- 
tributed to accurate results by introducing a correction for diurnal varia- 
tion, and thought that the fact that different corrections, when applied to 
different series of observations, brought the ultimate result to uniformity 
was a sufficient guarantee for their accuracy. 
The Hon. Secretary then read a paper by Mr. J. Josselyn Hanson, 
on Hydrse, or Freshwater Polypes. After referring to the history of our 
knowledge of this animal, the author continued : — 
The- true Zoophytes are divided into two classes — the Actinozoa and the 
Hydrozoa, the sea-anemone being taken as a type of the former, and the Hydra, 
or fresh=water Polype, of the latter. It is upon the animal typical of the Hydro- 
zoa and the simplest of the Zoophytes that I would offer a few remarks. 
In this country there are two species common, Hydra viridis, or green polype, 
and H. vulgaris, which is said to be usually of a dusky orange colour, although 
this would seem to vary with the nature of its food. The former of these is the 
only one I have ever been able to find during a search extending over a period of 
now nearly five years, and notwithstanding that this polype is frequently met with, 
yet in my experience it is not by any means to be found in every pond and ditch, 
as certain writers would make one believe. 
The diagnosis of the Hydra is as follows :— A single locomotive polype consti- 
tutes the animal ; it has tentacles, and a hydrorhiza ; the reproductive organs 
appear as simple processes of the body-wall. The only parts incapable of repro- 
ducing are the tentacles. * * * * The following remarks more especially 
apply to Hydra viridis, or green polype, as it is the only species in which I have 
had the opportunity of verifying the observations of others. The body of the 
polype is constantly assum ing a variety of forms ; sometimes when it is attached 
by its disc with arms extended fishing for its prey, it bears a close resemblance to 
certain of the fresh water algge, and when searching for it in my aquarium, I have-.; 
frequently only been undeceived by its suddenly contracting; then at other times 
it may be seen as a minute spherical body adhering to the glass, the arms being 
scarcely observable, and this is the form it generally assumes after having taken its 
prey. The hydrse move from place to place <c by fixing alternately the extremities 
