DYN 
is on these principles (which involve an 
immense collection of cases, and cir- 
cumstances), that the science of astrono- 
my, and whatever relates to the wonder- 
ful correspondence we observe in all the 
opemtions of the grand universal system, 
is founded. 
In rotatory motions we are always to con- 
sider, that eveiy atom which is at I'est re- 
quires a certain power to cause its removal; 
and, that w hen one part of a wheel moves, 
the whole must move ; therefore the power 
must be such as is equal to move the whole. 
Hence we find, tliat in a well-balanced 
wheel the motion is easy, because there are 
as many atoms disposed to descend as there 
are to be raised ; consequently, the opposing 
atoms are held in equilibrio. We must 
observe, how'ever, that the resistance to 
motion is greater as we approach the centre ; 
for a power which w’ould give a wheel mo- 
tion when applied at its perimeter, or exte- 
rior, w'ould be inadequate to set it in action 
if applied near to the axis. Therefore 
powers applied at the greatest distance from 
the centre, have more force than such as are 
applied nearer to the centre : their eflfeqts 
will be in exact ratio with the squares of 
their distances from the centre, while the 
imparted velocity will diminish in exact 
proportion with the accession of force. Of 
this we see innumerable instances in clocks, 
cranes, and other machines, in which one 
wheel is made to move another; or in any 
system of wheels. We cannot, indeed, have 
a more familiar demonstration than is af- 
forded by the greater facility with wliich the 
hind wheel of a coach revolves, compared 
with the fore wheel, which, being so much 
smaller, has the power (i. e. the earth) so 
much nearer its axis, and consequently re- 
volves with an increase of velocity propor- 
tioned to its difference of diameter. 
Before we quit this article it may be pro- 
per to observe, that the principles of gyra- 
tion and of oscillation have a close connec- 
tion with the foregoing points. The powers 
of windlasses, winches or cranes, jacks, &c. 
all depending upon the application of a 
power at more, or less, distance from the 
centre. Thus we find the common steel- 
yard is affected by the removal of the pea, 
or shifting resistance, along a scale, where- 
on the power is indicated to augment, ac- 
cording as it recedes from the point of os- 
cillation. But we see that in scales equally 
removed from that centre, the perpendicu- 
lar distances of the weight, or of the goods 
to be weighed, do not in any degree change 
DYT 
the power, when the two points of suspension 
are equidistant from the centre of oscilla- 
tion ; and that the two scales, together 
with their suspending cords, &c. are per- 
fectly counterbalanced. A reference to 
fig. 10. will exhibit that, provided the two 
arms, or suspending points, A A, be 'equally 
removed from the point of oscillation, C, it 
matters not whether the scales be at equal 
distances below A A respectively, or whe- 
ther one scale be at D and the other at E, 
provided all their respective parts be per- 
fectly equipoised ; but if one arm should be 
longer, so as remove one scale further from 
the centi'e of oscillation, by giving unequal 
distances, C A and C F, between the two 
parts of suspension, their state of equili- 
brium would be thereby totally destroyed. 
We shall now finally observe, tliat in 
every branch of mechanics it will be found, 
that equable motion is the surest, the 
safest, and the most durable ; and that, in 
proportion as the forces, and the resistances 
thereto, are broken or fluctuating, so will 
the former be diminished and the latter be 
increased. Hence experience shews ns, 
that windmills wear more than water-mills, 
and that animal pow’ers are apt to tear ma- 
chinery to pieces. We can command an 
uniform supply of force where water is the 
power; but hitherto no means have been 
found so completely to regulate eitlier the 
quantity of wind, or the paces of cattle. 
DYNASTY, among ancient historians, 
signifies a race or succession of kings of the 
same line or family : such were the dynas- 
ties of Egypt. The Egyptians reckon tlrirty 
dynasties within the space of 36525 years ; 
but the generality of chronologers look 
upon them as fabulous. And it is very cer- 
tain that these dynasties are not continually 
successive, but collateral. 
DYSENTERY. See Medicine. 
DYSOREXY, among physicians, denotes 
a want of appetite, proceeding from a 
weakly stomach. 
DYSPEPSY, a difficulty of digestion, for 
which physicians prescribe bitters. 
DYSPNOEA, a difficulty of breathing, 
usually called asthma. 
DYSURY, in medicine, a difficulty of 
making urine, attended with a sensation of 
heat and pain. It is distinguished from a 
stranguary, as in tlie last, the urine is voided 
by only a drop, as it were, at a time, but, 
however, with pain ; and from an ischury, 
as ill this disorder, there is an almost total 
suppression of urine. 
DYTISCUS, in natural history, a genus 
