BALANCE, 
which weight, increafing in power as it afeends along the 
arc F G of a circle, and pointing by an index to the num¬ 
ber or divifion of the arc which denotes the weight of any 
■foody put into the fcale at E. And thus one conftant 
weight ferves to weigh all others, by only varying the po¬ 
fition of the arms of the balance, inftead of varying the 
places or points of fufpenfion in the arms themfelves. 
AJfay-Balance. This is a very nice balance, ufed in de¬ 
termining the exact weights of very fmall bodies. Its 
flrufture is but little different from the common fort; ex¬ 
cept that it is made of the belt and hardeft fteel, and made 
to turn with the fmalleft weight. For the conftrudtion of 
balances, on principles of great nicety and truth, Mr. Ni- 
cholfon, in his excellent Dictionary of Chemittry, gives the 
following rules and obfervations : “ The balance is a lever 
%vhofe axis of motion is formed with an edge like that of 
a knife; and the two dilhes at its extremities are hung up¬ 
on edges of the fame kind. Thefe edges are firft made 
fharp, and then rounded with a fine hone, or a piece of 
'buff leather. The excellence of the infirument depends, 
in a great meafnre, on the regular form of this rounded 
part. When the lever is confidered as a mere line, the 
two outer edges are called points of fufpevfon , and the in¬ 
ner the fulcrum. The points of fufpenlion are fuppofed to 
be at equal difiances from the fulcrum, and to be preffed 
with equal weights when loaded; its a£tion will then be 
determined by the following rules: 
“ i. If the fulcrum be placed in the centre of gravity 
of the .beam, and the three edges lie all in the fame right 
line, the balance will have no tendency to one polition 
more than another, but will reft in any polition it may be 
placed in, whether the feales be on or off, empty or loaded. 
“2. If the centre of gravity of the beam, when level, 
be immediately above the fulcrum, it will overfet by the 
fimallefi action ; that is, the end which is lowefi: will de¬ 
scend ; and it will do this with more fwiftnefs, the higher 
the centre of gravity, and the lefs the points of fufpenfion 
are loaded. 
tl 3. Bur, if the centre of gravity of the beam be im¬ 
mediately below the fulcrum, the beam will not refi in any 
pofition but when level; and, if difiurbed from that po- 
firicn, and then left at liberty, it will vibrate, and-.at lafl 
come to refi on the level. Its vibrations will be quicker, 
and its horizontal tendency ftronger, the lower the centre 
of gravity, and the lefs the weight upon the points of fuf¬ 
penlion. 
“ 4. If the fulcrum be below the line joining the points 
of fulpenfion, and thefe b^loaded, the beam will overfet, 
din lefs prevented by the w'Sight of the beam tending to 
produce an horizontal polition, as in rule 3. In this laft 
oafe, fmall weights will equilibriate, as in-rule 3 j a cer- 
Vol. II. No. 94. 
&J.I 
tain exadl weight will refi in any pofition of the beam, as 
in rule 1; and all greater weights will cauie the beam to 
overfet, as in rule 2. Many feales are often made this 
way, and will overfet with any confiderable load. 
“5. If the fulcrum be above the line joining the points 
of fufpenlion, the beam will come to the horizontal pofi¬ 
tion, unlefs prevented by its own weight, as in rule 2. If 
the centre of gravity of the beam be nearly in the fulcrum, 
all the vibrations of the loaded beam will be made in times 
nearly equal, unlefs the weights be very fmall, when they 
will be flower. The vibrations of balances are quicker, and 
the horizontal tendency fironger, the higher the fulcrum. 
“ 6. If the arms of a balance be unequal, the weights ini 
equipoife will be unequal in the fame proportion. It is a 
fevere check upon a workman to keep the arms equal, 
while he is making the other adjultments in a (Long and 
inflexible beam. 
“ 7. The equality of the arms of a balance is of life, in 
fcientific purfuits, chiefly in making of weights by bifec- 
tion. A balance with unequal arms will weigh as accu¬ 
rately as another of the fame workmanlhip with equal arms, 
provided the ftandard weight itfelf be firft.counterpoifed, 
then taken out of the fcale, and the thing to be weighed, 
be put into the fcale, and adjufted againft the counterpoife ; 
or when proportional quantities only are confidered, as iit 
chemical and other philofophical experiments, the bodies 
and produ6ls under examination may be weighed againft 
the weights, taking care always to put the weights into 
the fame fcale. For then, though the bodies may not be 
really equal to the weights, yet their proportions among!!: 
each other may be the fame as if they had been accurate¬ 
ly fo. 
“ 8. But, though the equality of the arms may be well 
difpenfed with, yet it is irrdifpenfibly necelfary that their 
relative lengths, whatever they may be, Ihouid continue 
invariable. For this purpofe, it is neceflary, either that 
the three edges be all truly parallel, or tliat the points of 
fulpenfion and fupport fliould be always in the fame part 
ot the edge. This laft requilite is the moll eafily obtain¬ 
ed. The balances made in London are ufually conftrucl- 
ed in fuch a manner, that the bearing parts form notches 
in the other parts of the edges; fo that, the feales being - 
fet to vibrate, all the parts naturally fall into the fame 
bearing. The balances made in the country have the ful¬ 
crum-edge ftraight, and confined to one conftant bearing 
by two (ide-plates. But the points of fufpenfion are re¬ 
ferred to notches in the edges, like the London balances. 
The balances here mentioned, which come from the coun¬ 
try, are inclofed in a fmall iron japanned box, and are to be 
met with at the Birmingham and Sheffield warehonfes. 
They are, in general, well made and adjufted, turn with the 
twentieth of a grain when empty, and will fenlibly (hew the 
tenth of a grain with an ouneb in each fcale. Their price 
is from 3s. to half-a-guinea; but thole which are under 
7s. have not their edges hardened, and confequently are 
not durable. This may be afeertained by the purchafcr, 
by palling the point of a pen-knife acrofs the fmall piece 
which goes through one of the end-boxes; if it makes any 
mark, or impreflion, the part is foft. 
“ 9. If a beam be adjufted fo as to have no tendency to 
any one pofition, as in rule 1, and the feales be equally 
loaded; then, if a fmall weight be added in one of the 
feales, that balance will turn, and the points of f ufpenfion 
will move with an accelerated motion, limilar to that of 
falling bodies, but as much flower, in proportion, very 
nearly, as the added weight is lefs than the whole weight 
borne by the fulcrum. 
“ jo. The ftronger the tendency to an horizontal pofi¬ 
tion in any balance, or the quicker its vibrations, as in 
rules 3 and 5, the greater additional weight will be requi¬ 
red to cauie it to turn, or incline to any given angle. No 
balance therefore can turn fo quick as the motion dedu¬ 
ced in rule 9. Such a balance as is there delcribed, if it 
were to turn with the ten-thoufandth part of the weight, 
would move at quickeft ten thoufand times flower tha« 
8 A falling 
