16G 
GAGE. 
and of different kinds of trees, and repeated the experi- comparifon may be made betwixt the ftrength of one gale 
ment in a variety of inffances. See his Vegetable Statics, of wind and that of another. 
vol. i. chap.ii. p. 84, &'c. The force of the wind may be likewife meafnred with 
5. The Tide-GagE. An inffrnment tifed for deter- this inffrument, by filling it until the water rims ont at 
mining the height of the tides by Mr. Bayly. in the conrfe -the hole G. For, if we tiien hold it up to the wind as 
of a voyage towards the fouth-pole, &c. in the Refolution before, a quantity of water will be blown out f and if 
and Adventure, in 1772, 1773, 1774, and 1775. This in- both legs of ilic inffrument are of the fame bore, the 
firument confifrs of a glafstube, w hqfe internal diameter 
was feven-tenths of an inch, laflied fad to a ten-feet fir 
rod, divided into feet, inches, and quarters: this rod was 
fjflened to a ftrongpoff fixed upright and firm in the wa¬ 
ter. At the lower end of the lube was an exceeding I'niall 
aperture, through which the water was admitted, dn 
confequence of this conflriiction, the furface of the water 
HI the tube was fo little affedted by the agitation of the 
fea, that its height was not altered one-tenth of an inch, 
when the'fvvell of the fea was two feet; and Mr. Bayly 
was certain, that wifh tltis inffrument he could difeern a 
difference of one-tenth of an inch in the height of the tide. 
6. Tlie Wind-G.age ; an inffrument for meafuring,the 
force of the wind upon any given furface. It was invent¬ 
ed by Dr. Lind, and firft deferibed in the Pliil. Tranf. 
vol. Ixv. The inflruiiient conliffs of two glafs tubes, of 
five or fix inches in length, as lliewn by the letters 
ABCD, in the engraving at fig. 5. Their bores, which 
are the better for being equ.il, are about four-tenths of an 
inch in diameter. They are connected together like a 
fiphon, by a final! bent glafs-tube marked ah, the bore 
of which is about one-tenth of an inch in diameter. On 
the upper end of the leg AB there is a tube of latten 
brafs, which is bent perpendicularly ■outward.^, and has 
its mouth open towards F. On the o.ther leg C D is a 
cover with a round hole G in the upper part of it, two- 
tenths of an inch in diameter. This cover and the bent 
tube are connetted together by a flip of brafs cd, which 
not only gives fliength to the whole inffrument, but alfo 
ferves to hold the (dale H I. The bent tube and cover 
are fixed on with hard cement or fealing-wax. To the 
fame tube is foldered a piece of brafs e, w ith a round hole 
to receive the flee! fpindle KL ; and aty there is juft fucii 
another piece of brafs foldered to the brafs-hoop g/t, 
which furroundsiboth legs of the inftrument. There is a 
fmall fnoulder on the fpindle at f, upon which the inlfrii- 
Mient refls, and a final! nut at i, to prevent it from being 
blown off the fpindle by the wind. The wliole inftrument 
3S ealily turned upon t!ic fpindle by th.e wind, fo as always 
to prelent the mouth of th.e bent tube towards it. The 
end of the fpindle has a ferew on it, by which it may be 
ferewed into the top of a poll, or a ftand made on piirpofe. 
It has alfo a hole at L, to admit a fmall lever.for ferew- 
ing it into wood with more facility. A thin plate of brafs 
k is foldered to the bent tube, about half an inch above 
the round hole G, fo as to prevent rain from falling into 
it. There is likewife a crooked tube, as (hewn at AB, 
ficr. 6, to be put occafionally upon the mouth of the bent 
tube F, in order to prevent rain from being blown into 
the moutli of the wind-gage wlien it is left out all night, 
orexpofed in th.e time of rain. 
The force or momentum of the wind is afeertained by 
this inftrument, by filling the tu'bSs'lialf full of water, 
and pulhing tlie fcale a little up or down, till the o of the 
fcale, when the inftrument is held up perpendicularly, be 
on a line with the furface of the water in both legs of the 
wind-gage. '1 he inftruiuent being thus adjuffed, hold it 
up perpendicularly, and, turning tlie mouth of the bent 
tube tovvardS' the wind, obferve how much the water is 
deprefl'ed by it in the one leg, and raifed in the other. 
The fum-of tlie two is the height of a column of water 
which the wind is-captible of luftaining at tb.at time; and 
every body tl’.it is opuofed to that wind will be preft'ed 
upon by a fcrce equal to the weight of a column of w a¬ 
ter, having its hafe equal to the.altitude of the column ot 
water lulftiji.ed by the wii-d in the wind-gage. . Hence.the 
{cree of the wind upon ,any body where the (urface 'op- 
height of the column fuff lined will beeqnal to double the 
column of water in either leg, or the fum of what is 
wanting in both legs. But it the legs are of unequal 
bores, neither of thefe will give the true lieight of the 
column of w'ater which the wind fuftained. But the true 
height may be obtained by the following formulae. 
Suppofe that after a gale of wind which had blown tlie 
water from A to B, in fig. 7, forcing it at the ftme time 
through the other tube out at E, the furface of the. water 
ftioutd be found ftanding at fome level as D G, audit 
were required to know what w as the height of the column 
E F, or AB, which the wind fuftained. Jn order to ob¬ 
tain this, it is only neceffary to find the lieight of the co¬ 
lumns DB, or G F, which are conftantly equal to one 
another; for either of thefe, added to one of the equal 
columns AD, EG, will give the true lieight of tlie co¬ 
lumn of water which the wind fuftained. Let the diame¬ 
ters A C, EH, of the tubes, be refpeftively reprefented 
by cd\ and let « A D or E G, and D B, or G F : . 
then it is evident, that the cohimrvD B is to the column 
E G as c2.x to But tliefe columns are equal. There¬ 
fore al-a ; and coqfequently x— ^ . But if, at 
c- 
any inftant of time wliilft tlie wind was blowing, it was 
obferved, that when the water flood at E, the top of the 
tube out of-whicli it is forced, it was depreffed in the other 
to fome given level B F, the altitude at which it would 
have flood in each had it immediately fubfided, may be 
found in the following manner. Let^=AB, or E F'. 
Then it is evident that the'column D B is equal to the dif¬ 
ference of columns E F, GF. But the difierence of thefe 
d^b 
columns is as d'^b — d'^x ; and confequently xz :^—^— 
For the cafes when the wind blovv.s in at the narrow le^ 
mf the inftrument : Let A B = E F —h, E G or A D =2 
a, G F =: D B — X, and th.e diameters E 11 , G A, refpec- 
tively —d, c, as before. Then it is ev ident that the co¬ 
lumn A D is to the column G F as tod^x. But thefe 
columns are equal; tlierefore d'^xz=.ac^-, and cenfe- 
quently x — —^- It is alfo evident, that the coluirm AD 
is equal to the difference of the columns A B, D B ; but 
the difference of thefe columns is as —c^x. Thgre- 
hc'^ 
{oxt d'^xzzzbc'^ — c~x. Whence we get x c_ 
The life of the fmall tube of communication a-b, fig. 5., 
istocheck the undulation of the waver, fothat the lieigW 
of it may he read off from the fcale with eafe and certain¬ 
ty. But it is particularly defigned to prevent the'wa-ter 
from being thrown up to a much greater or I'efs altitwde, 
than the true height of the column wliich the wind is able 
at that time to iuftain, from its receiving a fudden ira- 
puHe whilft it is vibrating either in itsafeent or defcent.. As 
in fome cafes frelh water in this inftrument might be liable 
to freeze, and thus break the tubes, Dr. Lind recom¬ 
mends a faturated folution of fea-falt to be ufed inftead 
of it, which does not freeze till'Fahrenheit’s^ thermome¬ 
ter falls to o. ' : 
7.- Gage of the Air-Pump. Thi§ is adapted forffiew- 
inv the degree to which the air is rarefied, or the receiver 
ib'exhaufled, at any lime by the air-pump. There are 
three fuch^gages.in ufe; the firft, called the hng bdromc- 
ier gage-, tlm fecond, \\\e;j!:ort larometer gage; the third, 
the pear gage, or mancifieter. The two firft fhew the elal,- 
ticiiy of the fluid in the receiver, buj: 
do not determine 
pofed to it is known may be eakiy found ; and a ready whether it .be permanently eiadicair; the third fli^wsUie 
