PHR 
measured by means of a graduation, the aerO 
is situated towards tiie top of the branch 
that is terminated by the enamelled ball. 
The use of this instrument is founded upon 
the principle, that when tlie light is ab- 
sorbed by a body, it produces a heat pro- 
portional to the quantity of absorption. 
When the instrument is exposed to the 
solar rays, those rays tliat are absorbed by 
the dark colour, heat the interior air, which 
causes the liquor to descend at first with 
rapidity in the corresponding branch. But 
as a part of the heat which had introduced 
itself by means of the absorption is dissipat- 
ed by the radiation, and as the difference 
between the quantity of heat lost and 
that of the heat acquired goes on dimi- 
nishing, there will be a point where these 
two quantities having become equal, the 
instrument will be stationary, and the inten- 
sity of the incident light is then estimated 
by the number of degrees which the liquor 
has run over. The author of this ingenious 
instrument has pointed out its advantages 
in determining the progressive augmenta- 
tion undergone by the intensity of the light, 
and the gradation in a contrary sense which 
succeeds to that progress, both from the 
beginning of the day to its end, and from 
the winter solstice to the end of the suc- 
ceeding autumn. With the help of such 
an instrument one might also compare the 
action of rays of light in different countries 
of which some dart with sufficient con- 
stancy from a fine and serene sky, while 
others seem to be covered with a veil which 
dims and obscures their lustre. Mr. Leslie 
having proposed to himself, to measure the 
energy of the several coloured rays which 
compose the solar spectrum, caused a beam 
of light to pass through a prism of flint glass ; 
and the indications of the photometer pre- 
sented successively to the different parts of 
tlie spectrum, have furnished nearly for the 
relation, between the degrees of force of 
the blue, green, yellow, and red rays, that 
of the numbers 1, 4, 9, 16 ; a relation which, 
considered in the two extreme terms, is 
more than quadruple that which was sub- 
stituted for it by Dr. Herschel, who has 
made experiments for the same purpose. 
PHRYGANEA,in natural history, a ge- 
nus of insects of the order Nenroptera. Ge- 
neric cbaratter : mouth with a horny short 
curved mandible ; feelers four; three stem- 
mata ; antenn® setaceous, longer than the 
thorax ; wings equal, incumbent, the lower 
ones folded. There are nearly sixty species 
in two divisions, A. Tail with two trun- 
VOL. V. 
PHR 
Cdte bridles. B. Tail without bristles. The 
insects of this genus hie seen in a summer’s 
evening floating in the air in large masses, 
and are eagerly devoured by Swallows. 
They resemble moths, particularly the 
division called Tine® ; but may readily be 
distinguished by their feelers, and also by 
the stemmata situated at the top of the 
head. The phrygane® proceed fr om aqua- 
tic larv® of a lengthened shape, residing in 
tubular cases, which they form by aggluti- 
nating various fragments of vegetable sub- 
stances, &c. These tiibidar cases are lined 
within by a tissue of silken fibres, .and are 
open at each extremity. The included 
larv®, when feeding, protrude the head and 
fore-parts of the body, creeping along the 
bottom of the waters they inhabit, by means 
of six short and slender legs ; on the upper 
part of the back, is a sort of prop, prevent- 
ing the case, or tube, from slipping too for- 
wwds during the time the animat is feed- 
ing. One of the largest species is the P. 
grandis, (see Plate IV. Entomology, fig. 2). 
This insect is about an inch in length, very 
like a phalmua ; the upper wings are grey, 
marked by various darker and lighter 
streaks and specks, and the under wings 
yellowish, brown, and semitransparent. The 
larv® of this genus is known by the name of 
cadpw-worm, and is frequently used by 
anglers as a bait. Wlmn arrived at full 
growth it fastens the case or tube by seve- 
ral silken filaments to the stem of some wa- 
ter plant, or other convenient substance, in 
such a manner as to project a little above 
the surface of the water, aud casting its 
skin, changes to a chrysalis of a lengthened 
shape, and displaying the immature limbs 
of the future phryganea, which in a fort- 
night emerges from its confinement. 
PHRYMA, in botany, a genus of the 
Didynamia Gymnospermia class and order, 
^fatural order of Personal®. Labials^ 
Jussieu. Essential character : seed one. 
There are two species, viz. P. leptostachya 
and P. dehiscens ; the former is a native of 
North America, the latter was found at the 
Cape of Good Hope by Thun berg. 
PHRYNIUM, in botany, a genus of the 
Monandria Monogynia class and order. Es- 
sential character : calyx three-leaved ; pe- 
tals three, equal, growing to the long chan- 
nelled tube of the nectary ; nectary tube 
filiform ; border four-parted ; capsule three- 
celled ; nuts three. There is but one spe- 
cies, viz. P. capitatum, which is anative of 
Malabar, China, and Cochin China, in shady 
moLst places, 
Y 
