W E I 



Weight, Pondus, in Mechanics, is any thing to be raifed, 

 fuftained, or moved by a machine ; or any thing that in 

 any manner refifts the motion to be produced. See Mo- 

 tion, &c. 



In all machines, there is a natural ratio between the 

 weight and the moving power. If the weight be increafed, 

 the power muft be fo too ; that is, the wheels, &c. are to 

 be multiplied, and fo the time increafed, or the velocity 

 diminifhed. 



" The centre of gravity F, ( Plate XL. Mechanics, Jig. 6. ) 

 of a body I H, together with the weight of the body, being 

 given ; to determine the point M, in which, lying on a ho- 

 rizontal plane, a given weight G, hung in L, cannot re- 

 move the body I H out of its horizontal fituation." 



Conceive a weight hung in the centre of gravity F, equal 

 to the weight of the whole body I H, and find the common 

 centre of gravity M, of that and the given weight G. If 

 the point M be laid on the horizontal plane, the weight G 

 will not be able to move the body H I out of its place. 



" 'The centre of gravity C {jig. 7. ) of a body A B, to- 

 gether with its weight G, being given ; to determine the 

 points L and M, wherein props M N and L O are to be 

 placed, that each may bear any given proportion of the 

 weight." 



In the horizontal line A B, pafling through the centre of 

 gravity C, alTume the right lines M C and C L in the given 

 ratio. Props, then, M N, L O, placed in thefe poijits, 

 will be prefled in the given ratio. 



Hence, if in the points M, L, in lieu of props, you 

 place the (houlders or arms of porters, &c. they will be able 

 to bear the burden alike ; if their (hares be proportioned to 

 their ftrengths. Thus we have a way of diftributing a bur- 

 den in any given ratio. 



Weights, Grofs, Neat, Penny, AJfay of, Ancel. See the 

 feveral articles. 



Weight of the Atmofphere. See Atmosphere. 



Weight of the Air, is equal to the elafticity thereof. 



To find the Weight of a Cubic Inch of Air. — Weigh a round 

 glafs vefTel full of common air, very accurately ; then ex- 

 hauft the air out of it ; weigh the exhaulled veffel, and fub- 

 tra£t the latter weight from the former, the remainder is the 

 weight of the air exhaufted. 



Find, then, tiie content of the veflel by the laws of mea- 

 furing ; and the ratio of the remaining air to the primitive 

 air. This done, the bulk of the remaining air is found by 

 the rule of three ; which being fubtrafted from the capacity 

 of the veflel, the remainder will be the bulk of air extradled. 

 Or, if the air-pump be very tight, and the exhauftion con- 

 tinued as long as any air is got out, the remaining air will 

 be fo fmall, that it may be very fafely neglefted, and the 

 content of the veflel taken for the bulk of the exhaufted 

 air. 



Having, therefore, the weight and bulk of the whole ex- 

 haufted air, the weight of one cubic inch is eafily had by the 

 rule of three. 



This method was firft ufed by Otto Guericke, and after- 

 wards by Burcher de Voider, who gives us the following 

 particulars in his experiment, i. That the weight of the 

 glafs fpherical veflel he made ufe of, full of common air, 

 was 7 lbs. I oz. 2 drs. 48 grs. ; when exhaufted of air, 7 lbs. 

 I oz. I dr. 31 grs. ; and when full of water, 16 lbs. 12 oz. 

 7 drs. 14 grs. The weight of the air, tl>erefore, was i dr. 

 17 grs. or 77 grs. ; the weight of the water 9lbs. I loz. 5 drs. 

 43 g""^- °'" 74743 grs. Confequently, the ratio of the fpe- 

 cific gravity between water and air is 74743 : 77 :: 970y-^ : 1. 

 Now, De Voider having found a cubic foot of water to 

 weigh 64 lbs., by inferring, as 970 is to 1, fo is 64 lbs. to a 



W E t 



fourth proportional, deduced by the rule of thrte, the 

 weight of a cubic foot of air, iiiz. i oz. 27 grs. or 507 grs. 

 nearly. Wolfii Elem. torn. ii. p. 291. 



From other later experiments accurately made with the 

 hydroftatical balance, a cubic inch of air appears to be 

 equal to two-fevenths of a grain, and therefore a cubic foot 

 equal to 4937- troy grains. There are various ways of efti- 

 mating the weight of the air ; for which, fee Air, Atmo- 

 sphere, Barometer, Specific Gravity, &c. 



It may be eafily determined by fitting a brafs cap, with a 

 valve tied over it, to the mouth of a thin bottle or Florence 

 flaflc, whofe contents are exaftly known, and fcrewing the 

 neck of this cap into the hole of the plate of the air-pump ; 

 then, having exhaufted the flafk of its air and taken it off 

 from the pump, fufpend it at one end of a balance, and 

 nicely counterpoife it by weights in the fcale at the other 

 end : when this is done, raife up the valve with a pin, and 

 the air will rufli into the flaflt, and caufe it to defcend. 

 When it is full of air, put grains into the fcale at the other 

 end to reftore the equilibrium ; and if the flafli holds exaftly 

 a quart, it will be found, that 1 7 grs. will be fufiicient for 

 this pnrpofe, when the quickfilver ftands at 29^ inches in 

 the barometer ; and this fhews, that when the air is at a 

 mean ratio of denfity, a quart of it weighs 17 grs.; and 

 confequently a gallon weighs 68 grs.: /'. <?. 231 cubic inches 

 of air are equal in weight to 68 grs., and 1728 cubic inches, 

 or a cubic foot of air, weighs 509^'; " grs. ; and as a cubic 

 foot of water weighs about 437702 troy grains, the fpecific 

 gravity of water will appear to be more than 850 times that 

 of air. See Air. 



The weight oi fea-ivater is different in different climates. 

 Mr. Boyle having furniflied a learned phyfician, going on 3 

 voyage to America, with an hydroftatical balance, and re- 

 commended him to obferve, from time to time, the difference 

 of weight he might meet withal ; this account was returned 

 him : that the fea-water increafed in weight, the nearer he 

 came to the line, till he arrived at a certain degree of lati- 

 tude, as he remembers, about the 30th ; beyond which, it 

 retained the fame fpecific weight, till he came to Barbadoes. 

 Philof. Tranf. N° 18. 



The weight of a cubical inch of good brandy, rum, or 

 other proof fpirits, is 235.7 &''^' > therefore, if a true inch 

 cube of any metal weighs 235.7 grs. lefs in fpirits than in 

 air, it ftiews the fpirits are proof; if it lofes lefs of its 

 aerial weight in fpirits, they are above proof; if it lofes 

 more, they are under : for the better the fpirits are, they 

 are the lighter ; and the worfe, the heavier. 



As all bodies expand with heat and contradl with cold, in 

 different degrees, the fpecific gravities of bodies are not 

 precifely the fame in fummer as in winter. It has been 

 found, that a cubic inch of good brandy is 10 grs. heavier 

 in winter than in fummer ; as much fpirit of nitre, 20 grs. ; 

 vinegar, 6 grs. ; and fpring-water, 3 grs. Hence it is moft 

 profitable to buy fpirits in winter, and fell them in fummer, 

 fince they are always bought and fold by meafure. It has 

 been found, that 32 gallons of fpirits in winter will make 

 33 in fummer. Fergufon's Left. p. 98. 4to. See Specific 

 Gravity, and Hydrometer. 



Weight of the Human Body. It is to be obferved, that 

 the heat and drynefs of the air both Icffen the weight of the 

 body, and the cold and moifture of the air both increafe 

 this weight. See Moisture. 



Much lleep, much food, and httle exercife, are the prin- 

 cipal things which increafe the weight of the body, and 

 make animals grow fat. Confequently, if the weight of the 

 body be too great for good and uninterrupted health, it 

 may be leffened by diminifliing fleep and food, and by in- 



creafing 



