DYN 



DYS 



(which involve an immense collection of 

 casi-sand circumstances), that the science 

 of astronomy, and whatever relates to the 

 wonderful correspondence we observe in 

 all the operations of the grand universal 

 system, is foxmded. 



In rotatory motions we are always to 

 consider, that every atom which is at rest 

 requires a certain power to cause its re- 

 moval : and that, when 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, that, 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, however, that the resistance to 

 motion is greater as we approach the cen- 

 tre ; for a power which would give a 

 wheel motion when applied at its peri- 

 meter, or exterior, would be inadequate 

 to set it in action if applied near to the axis. 

 Therefore, powers applied at the great- 

 est distance from the centre have more 

 force than such as are applied nearer to 

 the centre : their effects will be in exact 

 ratio with the squares of their distances 

 from the centre, while the imparted ve- 

 locity 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 

 afforded by the greater facility with which 

 the hind wheel of a coach revolves, com- 

 pared with the fore wheel, which, being 

 so much smaller, has the power (i. e. the 

 earth) so much nearer its axis, and con- 

 sequently revolves with an increase of 

 velocity proportioned to its difference of 

 diameter. 



Before we quit this article it may be 

 proper to observe, that the principles of 

 gyration and of oscillation have a close 

 connection with the foregoing points. The 

 powers of windlasses, winches, or cranes, 

 jacks, &c. all depending upon the appli- 

 cation of a power at more or less distance 

 from the centre. Thus we find the com- 

 mon steelyard is affected by the removal 

 of the pea, or shifting resistance, along a 

 scale, whereon the power is indicated 

 to augment, according as it recedes from 

 the point of oscillation. But we see, that, 

 m scales equally removed from that cen- 

 tre, the perpendicular distances of 

 the weight, or of the goods to be weigh- 

 ed, do not in any degree change the pow- 



er, wlien the two points of suspension 

 are equidistant from the centre of oscil- 

 lation ; and that the two scales, together 

 with their suspending chords, &c. are 

 perfectly 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 oscil- 

 lation, C, it matters not whether the scales 

 be at. equal distances below A A respec- 

 tively, or whether one scale be at D and 

 the other at E, provided all their respec- 

 tive parts be perfectly equipoised ; but if 

 one arm should be longer, so as to remove 

 one scale further from the centre of os- 

 cillation, by giving unequal distances, C 

 A and C F, between the two parts of 

 suspension, their state of equilibrium 

 would be thereby totally destroyed. 



We shall now finally observe, that 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 re- 

 sistances thereto, are broken or fluctu- - 

 ating, so will the former be diminished 

 and the latter be increased. Hence ex- 

 perience shews us, that windmills wear 

 more than water-mills, and that animal 

 powers are apt to tear machinery 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 either 

 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 

 dynasties of Egypt. The Egyptians rec- 

 kon thirty dynasties within the space of 

 36525 years; but the generality of chro- 

 nologers look upon them as fabulous. 

 And it is very certain that these dynas- 

 ties are not continually successive but 

 collateral. 



DYSENTERY. See MEDICINE. 



DYSOREXY, among physicians, de- 

 notes a want of appetite proceeding 

 from a weakly stomach. 



DYSPEPSY, a difficulty of digestion, fot 

 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 strangury, as in the last the urine is 

 voided by only a drop, as it were, at a 

 time, but, however, with pain ; and from 

 an ischury, as in this disorder there is an 

 almost total suppression of urine. 



