1829.] 
On Fresh Water Testctcea. 
363 
it is highly improbable to suppose that the instrument, should be deranged during 
the very short lime that elapses between the successive observations of the hot and 
of the cold column. 
We have included in the following table, the mean result of a great number of 
observations made in the manner just described. The first column contains the 
temperatures deduced from the air thermometer, the second the mean expansion of 
mercury between the temperature of melting ice, and each of the temperatures 
found in the first column. The third gives the temperatures, such as they would 
he, supposing the expansion of mercury uniform, or in other words, such as would 
be shown by a thermometer constructed with this fluid, the containing vessel of 
which should have the same rate of expansion as itself. 
Table 2. 
Temperatures as shown 
by an air thermometer. 
Mean expansions of 
mercury. 
Temperatures supposing- the 
expansion of mercury uniform. 
0 

0 
100 
ss'ss 
100 
200 
3 52 3 
204,61 
300 
33oo 
314,15 
IV. Observations on the Occurrence of Fresh Water Testacea in tem- 
porary Pools formed by Rain, and unconnected ivith permanent bodies 
of Water. 
[To the Editor of Gleanings in Science.] 
Sir, 
I wish to attract the attention of such of your readers, as may have opportunities 
and inclination to investigate the subject, to a fact which is of frequent occurrence 
in this country; and which claims notice from its extraordinary nature, although, I 
cannot find that it has yet been discussed. I allude to the faculty possessed by some 
animals formed for living in, and respiring water, of inhabiting places, which during 
a lar»e portion of the year are not visited by moisture ; and which, containing 
water during the season of the periodical rains only, have then no communication 
with any other body of water. From this fact, one of three conclusions seems 
inevitable either that the animal in its perfect state, has the power of retaining 
sufficient moisture within itself to snstain vitality until the return of rain ; that it 
is enabled to withstand the inclemency of the hot season, in a state of extreme 
dryness ; or that its ova possess the same extraordinary faculty. The occurrence of 
the common Planorbis, which is scattered so widely over these provinces in every 
water from the running stream to the smallest pool, in shallow tanks which are 
dried up during a large portion of the year, may not excite so much surprize, as it 
belongs to a division of trnchelipodes which breath air ; but the appearance in such 
situations of a species of Palu dints*, belong to a genus, which is known to breathe 
water only, cannot so easily be passed over. This species 1 have found in a ditch, 
at Banda, excavated for the enclosure of a compound, and carried across the slope 
of a rising ground without any attention to level, so that the ditch rises on both 
sides of a part where the rain waters collect from the slope of the ground enclosed. 
No water, but that supplied by rain, can possibly have access to ibis place. The 
superfluous waters fall down the slope beyond, whence, during very heavy rain, they 
proceeded to a ravine distant more than a quarter of a mile, where they fall per- 
pendicularly more than 20 feet, and are thence carried to the river; so that it is 
not possible, that shell fish should have access from the river by this channel. As 
* It is small, ovate-oblong, wliitish-yellow, sub-hyaline, the lip at the base of 
tlie columella slightly produced t operculum calcareous, it floats on the surface of 
water when it lias attained it by climbing, but sinks when alarmed. It has no ob- 
jection to dead animal substances a3 food 1 
