EVA 
EVA 
if we place between them a thick ring of 
elastic gum, we may represent the natural 
equilibrium between the forces of cohesion 
and of repulsion ; for the ring would resist 
any small additional pressure with the same 
force as would be required for separating 
the hemispheres, so far as to allow it to ex- 
pand in an equal degree ; and at a certain 
point the ring would expand no more, the 
air would be admitted, and the cohesion 
destroyed, in the same manner as when a 
solid of any kind is torn asunder. 
But all suppositions founded on these 
analogies must be considered as merely 
conjectural ; and our knowledge of every 
thing which relates to the intimate consti- 
tution of matter, partly from the intricacy 
of the subject, and partly for want of suffi- 
cient experiments, is at present in a state 
of great uncertainty and imperfection. 
ETHICS, or Morality, the science of 
manners or duty, which it traces from man's 
nature and condition, and shews to termi- 
nate in his happiness ; or, in other words, 
it is the knowledge of our duty and felicity, 
or the art of being virtuous and happy. See 
Moral Philosophy. 
ETHULIA, in botany, a genus of the 
Syngenesia Polygamia iEqualis class and 
order. Natural order of Composite Dis- 
coidean Corymbiferaj, Jussieu. Essential 
character: receptacle naked ; down none. 
There are six species. 
ETYMOLOGY, that part of grammar 
which considers and explains the origin and 
derivation of words, in order to arrive at 
their first and primary signification, whence 
Quintilian calls it originatio. See Gram- 
mar. 
EVAPORATION, in natural philosophy, 
is the conversion of water * into vapour, 
which in consequence of becoming lighter 
than the atmosphere, is raised considerably 
above the surface of the earth, and after- 
wards by a partial condensation forms 
clouds. It differs from exhalation, which 
is properly a dispersion of dry particles 
from a body. When water is heated to 
212°, it boils, and is rapidly converted into 
steam ; and the same change takes place 
in much lower temperatures ; but in that 
case the evaporation is slower, and the elas- 
ticity of the steam is smaller. As a very 
considerable proportion of the earth’s sur- 
face is covered with water, and as this 
water is constantly evaporating and mixing 
with the atmosphere in the state of vapour, 
a precise determination of the rate of eva- 
poration must be of very great importance 
in meteorology. Accordingly, many experi- 
ments have been made to determine the 
point by different philosophers. No per- 
son lias succeeded so completely as Mr. 
Dalton : but many curious particulars had 
been previously ascertained by the labours 
of Richman, Lambert, Watson, Saussnre, 
De Luc, Kirwan, ahd others. From these 
vve learn that, 1. the evaporation is confined 
entirely to the surface of the water : hence 
it is in all cases proportional to the surface 
of the water exposed to the atmosphere. 
Much more vapour of course rises in mari- 
time countries, er those interspersed with 
lakes, than in inland countries. 2. Much 
more vapour rises during hot weatiier than 
during cold : hence the quantity evaporated 
depends in some measure upon tempera- 
ture. The precise law has been happily 
discovered by Mr. Dalton, who says, in 
general, the quantity evaporated from a 
given surface of water per minute at any 
temperature, is to the quantity evaporated 
from the same surface at 212", as the force 
of vapour at the first temperature is to the 
force of vapour at 212°. Hence, in order 
to discover the quantity which will be lost 
by evaporation from water of a given tem- 
perature, we have only to ascertain the 
force of vapour at that temperature. Hence, 
we see that the presence of atmospheric 
air obstructs the evaporation of water; but 
this evaporation is overcome in proportion 
to the force of the vapour. Mr. Dalton 
ascribes this obstruction to the vis inertia 
of air. 3. The quantity of vapour which 
rises from water, even when the tempera- 
ture is the same, varies according to cir- 
cumstances. It is least of all in calm 
weather, greater when a breeze blows, and 
greatest of all with a strong wind. Mr. 
Dalton has given a table that shews the 
quantity of vapour raised from a circular 
surface of six inches in diameter in atmo- 
spheric temperatures. The first column ex- 
presses the temperature ; the second the 
corresponding force of vapour ; the other 
three columns give the number of grains of 
water that would be evaporated from a 
surface of six inches in diameter in the 
respective temperatures, on the supposition 
of there being previously no aqueous vapour 
in the atmosphere. These columns present 
the extremes, and the mean of evaporation 
likely to be noticed, or nearly such; for 
the first is calculated upon the supposition 
of 35 grains loss per minute, from the 
vessel of 3* inches in diameter ; the second 
45, and the third 55 grains per minute. 4. 
