MECHANICS. 
737 
fmooth Sat plats; and EF and G H two rulers or flat 
pieces, a quarter of an inch thick, and twenty-four inches 
long, fixed flat upon the plate. At the lower part or be¬ 
ginning of the fcale, FH, the rulers are ^ an inch afun- 
der, and at the top, EG, of an inch : thus they in¬ 
clude between them a long converging canal, which is 
divided on one fide into a number of frnall equal parts, 
and which may be confidered as performing the offices 
both of the tube and fcale of the^pommon thermometer. 
It is obvious, that if a body, I, fo adjufted as to fit exactly 
at the wider end of this canal, be afterwards diminiffed 
in its bulk by fire, as the pyrometer-pieces are, it will 
then pafs further up the canal, and more and more fo ac¬ 
cording as the diminution is greater ; and converfely, that 
if a body, fo adju(ted as to pafs on to the narrow end, be 
afterwards expanded by fire, as is the cafe with metals, 
and applied in that expanded ftate to the fcale, it will not 
pafs fo far; and that the divifions on the fide will be the 
meafures of the expanfions of the one, as of the contrac¬ 
tions of the other, reckoning in both cafes from that 
point to which the body was adjufted at firft. 
Mr. Wedgwood connected the fcale of his pyrometer 
with Fahrenheit’s thermometer. The firft degree of his 
l'cale, which marks a red heat, correfponds to 947 0 Fah¬ 
renheit ; but, to make this inftrument better underftood, 
we may ftate a fevv of the conefponding degrees of the 
two inftrumenls. 
Wedgwood. 
- o : 
- 28 
3 s 
- 95 
1 3 ° 
169 
Fahrenheit. 
: 947 
4717 
52 37 
13427 
17977 
21877 
*40 
32277 
Red heat 
Fine filver melts - 
Fine gold melts - 
Welding heat of iron - 
Gaft iron melts - 
Greateft heat in an air-furnace? 
eight inches fquare - - J 
Extremity of the fcale, or higheft? 
temperature obferved - - i 
This inftrument has been of confiderable importance 
in fome arts and manufactures, and it is undoubtedly 
fitted to give fome information concerning thofe intenfe 
heats which c^ut be meafured by no other inftrument 
which has yet been contrived. But, as the fame kind of 
clay cannot always be obtained, and as it is probable that 
the contractions of the cylinders are not proportional to 
the temperatures, their eftimation by this inftrument can 
only be con-fidered as an approximation to certainty. 
And indeed it is proper to obferve, that M. Fourmy has 
lately given, in the Journal des Mines, a paper ‘ e On the 
Thermometers of baked Earths, termed Pyrometers,” in 
which he attempts to (liow that the effect of (hrinking, 
upon which Wedgwood’s pyrometer is founded, does not 
refitlt folely and invariably from the caufe to which it is 
afcribed ; that it is not necefiarily proportionate to it; 
that, whatever may be the graduation and the continuity 
of temperature applied to an aluminous mixture, its 
fhrinking is not only not neceflarily graduated, nor ne- 
ceffarily continuous, but it alfo does not always necefla¬ 
rily take place; and therefore that a pyrometer founded 
upon fuch (hrinking does not afford fo conftant and ac¬ 
curate a meafure tor the higheft degrees of heat as the 
dilatation of mercury or of alcohol does for the lower. A 
tranflation of M. Fourmy’s obfervations is inferted in the 
Repertory of Arts, No. 38. N. S. 
Hygrometers for meafuring the changes of the air in re- 
fpeCt to moillure and drynefs, have been fully defcribed 
and figured under the article Hygrometry, vol. x. 
p.631-636. 
Of Winds, Sound, Vapours, and the Formation of 
Springs, 
Wind is a current of air; and its direction is denomi¬ 
nated by that point of the compafs from which it blows. 
The principal if not the only caufe of wind, is a partial 
rarefaflion of the air by heat. When the air is heated, 
it becomes rarer, and therefore afcends; and the furround- 
V01*. XJV. No, ioo a . 
ing cold air, ruffling in to fupply its place, forms a cur¬ 
rent in fome one direction. Winds may be divided into 
conftant, or thofe which always blow in the fame direflion ; 
periodical, or thofe which blow half a year in one direction, 
and half a year in the contrary direflion; thefe are called 
monfoons and variable, which are iubjeCt to no rules. The 
two former are alfo called t^We-winds. We (hall here 
give the principal phenomena of winds, from Dr. Halley’s 
Hiftory thereof, in the Philofophical Tranfaftions. 
ift. In the Atlantic and Pacific Ocean, under the equa¬ 
tor, there is a conftant eaft wind. 
2dly. To about 28° on each fide of the equator, the 
wind on the north fide declines towards the north-eaft ; 
and the more fo the further you recede from the equator: 
and on the fouth fide, it declines in like manner towards 
the fouth-eaft. The limits of thefe winds are greater, in 
the Atlantic Ocean, on the American than on the African 
fide, extending in the former cafe to about 32 0 , and in 
the latter to about 28°. And this is true likewife to the 
fouthward of the equino&ial; for, near the Cape of Good 
Hope, the limits of the trade-winds are 3 0 or 4 0 nearer 
the line than on the coaft of Brafil. 
3dly. Towards the Caribee illands, the aforefaid north- 
ealt wind becomes more and more eafterly, fo as fometimes 
to be eaft, and fometimes eaft-by-fouth, but moftly north¬ 
wards of the eaft, a point or two. 
4thly. On the coaft of Africa, from the Canaries to 
about io° north latitude, the wind fets in towards the 
north-weft ; then it becomes fouth-weft, approaching more 
to the fouth as you approach the Cape. But, away from 
the coafts, the winds are perpetually between the fouth 
and the eaft; on the African fide they are more foutherly; 
on the Brafilian, more eafterly, fo as to become almolt 
due eaft. Upon the coaft of Guinea, they are fubjeCt to 
frequent calms, and violent gufts, called tornados, from 
all points of the compafs. 
5thly. In the Indian Ocean, the winds are partly con¬ 
ftant and partly periodical. Between Madagafcar and 
New Holland, from io° to 30 0 latitude, the wind blows 
fouth-eaft by eaft. During the months of May, June, 
July, Auguft, September, Offlober, the aforefaid fouth- 
eaft winds extend to within 2 0 of the equator; then, for 
the other fix months, the contrary winds fet in, and blow 
from 3 0 to io° fouth latitude. From 3 0 fouth latitude 
over the Arabian and Indian Teas, and Bay of Bengal, 
from Sumatra to the coaft of Africa, there is another 
monfoon, blowing from Oftober to April on the north- 
eaft point, and in the other half year in the oppofite di¬ 
rection. Between Madagafcar and Africa, a fouth-fouth- 
weft wind blows from April to October, which, as you 
go more northerly, becomes more wefterly, till it falls in 
with the weft-fouth-weft winds; but the doCfor could not 
obtain a fatisfaCtory account how the winds are in the 
other half-year. To the eaftward of Sumatra-and Ma¬ 
lacca, on the north fide of the equator along the coaft of 
Cambodia and China, the monfoons blow and change at 
the fame time as before mentioned ; but their directions 
are more northerly and foutherly. Thefe winds reach to 
the Philippine Iftands and to Japan. Between the fame 
meridians, on the fouth fide of the equator, from Sumatra 
to New Guinea, the fame monfoons are obferved. The 
Ihifting of thefe winds is attendecl with great hurricanes. 
The eaft wind about the equator is thus explained. The 
fun moving from eaft to welt, the point of greateft rare¬ 
faction of the air, by the heat of the fun, muft move in 
the fame direction; and, the point of greateft rarefaction 
following the fun, the air muft continually rulh in from 
the eaft, and make a conftant eaft wind. 
The conftant north-eaft wind on the north fide of the 
equator, and fouth-eaft wind on the fouth fide, may be 
thus accounted for. The air towards the poles, being 
denfer than that at the equator, will continually rulh to¬ 
wards the equator; but, as the velocity of the different 
parts of the earth’s lurface, from its rotation, increafes 
as .you approach the equator, the air which is raffing 
9 B , - from 
