DYN 



DYS 



^ which involve an immense collection of 

 cases and 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 founded. 



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 re- 

 sistance to motion is greater as we ap- 

 proach the centre ; for a power which 

 would give a wheel motion when applied 

 at its perimeter, or exterior, would be in- 

 adequate to set it in action if applied 

 near to the axis. Therefore, powers ap- 

 plied at the greatest distance from the 

 centre have more force than such as are 

 applied nearer to the centre : their ef- 

 fects will be in exact ratio with the 

 squares of their distances from the cen- 

 tre, while the imparted velocity will di- 

 minish in exact proportion with the ac- 

 cession offeree. Of this we see innumer- 

 able 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, compar- 

 ed 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 au increase of 

 velocity proportioned to its difference of 

 diameter. 



Before we quit this article it may be 

 proper to observe, that the principles of 

 gy ration and of oscillation have a close con- 

 nection 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 

 common steelyard is affected by the re- 

 moval of the pea, or shifting resistance, 

 along a scale, whereon the power is indi- 

 cated to augment, according as it recedes 

 from the point of oscillation. But we 

 see, that, in scales equally removed from 

 that centre, the perpendicular distances 

 of the weight, or of the goods to be 



weighed, do not in any degree change 

 the power, when the two points of sus- 

 pension are equidistant from the centre 

 of oscillation ; and that the two scales, to- 

 gether with their suspending chords, &c. 

 are perfectly counterbalanced. A refer- 

 ence to fig. 10 will exhibit, that, provided 

 the two arms, or suspending points, A A, 

 be equally removed from the point of os- 

 cillation, C, it matters not whether the 

 scales be at equal distances below A A 

 respectively, or whether one scale be at 

 D and the other at E, provided all their 

 respective parts be perfectly equipoised ; 

 but if one arm should be longer, so as to 

 remove one scale further from the centre 

 of osciliation, 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 dynasties 

 are not continually successive, but collu- 

 teral. 



DYSENTERY. See MEDICINE. 



DYSOREX Y, among physicians, denotes 

 a want of appetite proceeding from a 

 weakJy stomach. 



DYS PEPS Y, 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 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. 



