M E C H A N I C S. 
a barrel is 769^ lbs. The fecond column contains the cor- 
refponding fpecific gravity of the fame fluid. The figures 
exprefiing thefe numbers are of courfe inferred in the glafs 
tube, fo as to read on the fame line; and the whole ap¬ 
pearance of the inftrument may be readily imagined. 
But, fince the numbers on the inftrument proceed by 
tens, it will be neceflary to have auxiliary Tables for the 
intermediate pounds, and even for the ounces. 'I hefe 
we mud extract from Mr. Snart’s pamphlet. 
Table of Intermediate Numbers. | 
Pounds. 1 Ounces. 
I 
"0027 
I • 
•0001 
2. 
•0055 
2 
•0003 
3 
•0082 
4 
•0006 
4 
•01 1 O 
8 
•0013 
5 
•01 38 
6 
•0165 
7 
•0193 
S 
*0221 
9 
•0248 
Example. Suppofe the inftrument fiiould float in the 
given fluid at 625. Without calculation we know that 
625 lbs. is the weight of a barrel, or 36 gallons, of that 
fluid; but, in re (petit to its fpecific gravity ., we obferve 
that the neareft number on the item of the inftrument is 
622 lbs. of which the fpecific gravity is 1-7183. We muft 
therefore recur to the Table of Intermediate Numbers, and 
proceed thus: 
The fpecific gravity of 622lbs. is - 1-7183 
Ditto - - of 3lbs. is - -0082 
Specific gravity required, - 1-7265 
Though this inftrument is called a Chemical Aerometer, 
becaufe its fcale comprehends all the differences from 
water upwards, yet it is equally applicable to the pur- 
pofes of brewers, tanners, foap and glue boilers, fugar- 
refiners, lemon-acid anil vinegar makers, provifion-mer- 
chants, See. and, by a fimplification of the fcale, together 
■with a thermometer, may be made to fliow the leaft differ¬ 
ence, even to the 50th part of an ounce in a gallon. 
Fig. 17. reprefents another little portable inftrument con- 
Jtrufted by the fame ingenious gentleman. He has named 
it Ckondromctcr ; and it determines, without calculation, the 
weight, or fpecific gravity, of a buftiel of any kind of grain, 
feed, farina, &c. by a fmall fample, not more than the 448th 
part of a buftiel. The conftruftion and decifions of it are 
fo plain, that the neceflity of calculation, even when the 
weight of the buftiel is ever fo fractional, is totally pre¬ 
cluded, and made fubfervient to a mechanical procefs; 
for, the weight and meafure being both proportioned and 
practically reduced and equalized, no matter how oddly 
odd the weight of the bufliel may be, provided it be the 
8-gallon or Winchefter meafure, to which the inftrument 
is adapted; yet the 34. and 36 quarts, or 8§ and 9 gallons, 
which are uled in home parts of England, are as readily 
fliown by a trifling alteration in it. The inftrument is 
made either on a fmall fcale, and fitted into a box, to be 
carried in the pocket; or without a box, on a larger fcale, 
to be ufed in counting-iioufes. The fmaller machine is 
that reprefented on our Plate. The whole exterior di- 
menfions of the box are, 9 inches long, wide, and 2 
inches deep. A, the proportionate bufhel. B, the aim 
of the feale-beam. C, the divided fcale, on which the 
weight of the grain under trial is found. D, the move- 
able weight or index, which points out the fame. E, the 
jointed cheeks, or fulcrum, which lies down in the box 
with the inftrument when out of ufe. F, the roll- 
ftrike; f, the flat-ftrike. G, the mahogany box for the 
whole. ' 
When a fample of any article is to be proved, firft, 
fill the meafure or bucket in the fame careful manner 
you would do if it were a real buftiel, (without (baking 
it,) and with the roll ftrike off the top, only^obferving, 
as this inftrument is upon a diminifhed fcale, that, as the 
more bearded or rougher forts of grain (fuch as oats, Sec.) 
are more liable to be torn up by linking than the fmooth 
and free, they fliould be charged fomewhat heavier, or 
they might lofe too great a proportion in fo fmall a quan¬ 
tity by ltriking, and thereby appear too light. The mea¬ 
fure being filled, hook it on to the end of the feale-beam; 
then remove the brafs Hiding weight to that part of the 
divided fcale where it may be jutt preponderated by the 
bucket and its contents. Laftly,- look at the fiducial or 
bevelled edge of the brafs Aiding weight, and obferve at 
which degree it (lands, as that divifion will be the weight 
of an 8 gallon buftiel of the grain upon trial. If inftead 
of the roll you make ufe of the flat ftrike, the grain will 
appear to be from 1 to 2lbs. per bufhel lighter than if the 
former had been ufed. 
Of the RESISTANCE of FLUIDS. 
The refiftance of a body moving in a fluid arifes from 
the inertia, the tenacity, and fridion, of the fluid, admit¬ 
ting the particles to be in contadf. The latter caufe, 
granting it to exift, is probably very fmall ; and the fe- 
cond is, in moll fluids, inconfiderable when compared 
with the inertia. The refiftance therefore, which we (hall 
here confider, is that arifing from the inertia of the fluid. 
Prop. XVII. If a planeJurface move in a fluid with a ve¬ 
locity V in a airedion perpendicular to its plane , the reflflance, 
within certain limits of the velocity, varies as V 3 .—For the re¬ 
fiftance muft vary as the number of particles which the 
plane ftrikes in a given time, multiplied into the force of 
each againftthe plane. Now the number of particles which 
the plane ftrikes in a given time, muft evidently be in pro¬ 
portion to V ; alfo, the force of each particle is as V ; and, 
as aftion and re-aftion are equal and contrary, the re-ac¬ 
tion of every particle of the fluid again ft the plane muli 
be as V ; hence, the refiftance varies as V x V, or as V 2 . 
This is found, by experiment, to be very nearly true, when 
the velocity is fmall. 
This proof fuppofes, that, after the plane ftrikes a par¬ 
ticle, the aftion of that particle immediately ceafes, and 
the particle itfelf to be, as it were, annihilated ; but the 
particles, after they are (truck, mull necefiarily be made 
to diverge and aft upon the particles behind, which makes 
fome difference between this theory and experiment. 
Alfo, by increafing the velocity of the body, the action 
of the fluid behind it, to impel it in the direftion of its 
motion, will be diminiflied, and confequently the retar¬ 
dation will, on this account, be increafed. Mr. Robins 
found, from experiment, that, when a bullet moves with 
the velocity of found, or with a greater velocity (in which 
cafe, a vacuum is left behind the body, and the prefl’ure 
forwards from behind then ceafes), the refiftance is always 
greater than this law gives it. When bodies defeend in 
fluids, fuch as water, the refiftance is very nearly as V 2 , 
becaufe the body can never acquire a velocity beyond a 
certain limit. We will therefore, in the Articles here 
given upon refiftances, fuppofe the refiftance to vary as V' 3 . 
This law of refiftance was eflabliftied by fir I. Newton, 
from a variety of experiments ; fee the Principia, vol. ii. 
Prop. XXXI. Scholium : alfo Mr. Parkinfon’s Hydrofta- 
tics, p. 26. 
Prop. XVIII. When different planes move in diredions per¬ 
pendicular to their furfaces, in different fuids, and with dif¬ 
ferent velocities, the rcfflances will be as the Jquarcs of their ve¬ 
locities x the derfitics of the fluids X the areas of the planes.—- 
For, by increafing the denfity of the fluid, the number of 
particles (truck in the fame time will be greater in the 
fame proportion, and confequently the refiftance will, ca- 
tcris paribus, be greater in the fame ratio. Alfo, by in¬ 
creafing the area of the plane, the greater will be the num¬ 
ber of particles (truck in the fame ratio, and therefore the 
refiftance will be greater in the fame proportion. And, 
(Prop. XVII.) when the velocities vary, cateris paribus, 
the refiftance varies as V 2 . Hence, combining thele ratios 
together. 
