3 1 g; T he clearing away o f woods leffens the vapours,
and confequentïy,diminilhes the quantity o f rain, and
incréafés the temperature Several parifhes in Jamaica
-which ufed to produce fine crops; o f fugar canes, are
now dry for 9 months in. a. year and are turned into
cattle-pens, through .the clearing away o f the woods.
Hence, water is moft plentiful in thofe countries where
'woods, abound, and the bell fprings; are there found.
ïn'dmerica, Cnee the woods in the neighbourhood-of their
■ towns have been cut down, many ftreams have become
dry ; and others have been reduced fo low, as to caufe
great interruptions to the miller. , ’
320. O f evaporation, the following. fafts may be 9b-
ferved. 1 . T h a t in our climate's,' evaporation-is about
four times as great from the 21ft o f March to the 21ft
o f September; as from the 21ft o f September to the
2 lf t o f March.
12- T h a t, other circumftances being the fame, it is
greater in proportion as the difference between tlte temperature
o f the air, and that o f the evaporating furfaee
is greater ; and fo much thelinaller, as the 'difference-is
fmaller 5 and thereforehnaUeft, when the; temperature- o f
the air and evaporating liquor are equal. T h e former
part o f this propofftion however requires- fome ^eftric-
tion ; for i f air be more1 than,ï j degrçes colder than the
evaporating furfaee,. there, is fcarce any evaporation;
b,ut on the contrary, it depofits its moifture on the fur-
face o f the liquor.. : ;
3. T he degree o f cold produced b y evaporation, is always
much greater jwheri ' the air is warmer than the
evaporating furfaee, than that which ' is produced when
the furfaee is warmer than the air. Hence, warm winds,,
as. the Sirocco and Harmaian, are more drying than cold
winds.
4. Evaporation is more copious when 'the '.air isl-.lefs.
loaded with vapours, and is therefore greatly promoted
b y cold winds flowing into warmer countries.
5. Evaporation is greatly increased b y a current o f
air or wind flowing over the evaporating furfaee, be~
caufe unfaturated air. is conftantly brought into contaft
with it. Hence, calm days -are hotteft, as has commonly
been remarked;
-6. Trafts-of land covered-with trees or vegetables
emit more vapour, than the. famé fpace covered withwater*
M r. W 11. l 1A m s { Philadelphia Jratfaiii&ns^ found
this'quantity to amount.to;,^ more. Hence, the air about"
a -wood .or foreft is made colder by evapeuytubny from
'trees and ihrubs, while ymtMB^i'th^fch*es|a{Sg;^gpt,iti
a more moderate heat, ,and fecured from the burning
heat o f the fu n . by the yappurs .perfpired from the
leaves. Thus,, we-find the lhade o f vegetables more
effectual to cool us, as well as more'agreeable, than the
- lhade from rocks and buildings.
3 2 1 . 'T h e heat and cold o f different countries .are
tranfmitted from one to the other, by the medium o f
winds. - : .
322. From what has been‘observed it isrtnan*fe|^ that
fome fituations are ;b.etteiy&t»Ke&fc0 ^.^q^vejoticommunf-
cate-heat, than others^ , thus,'-h’igh-asmd.mountainous,
fituations being nearer to' the fohcceof-coKl than lower
fituations } and .countries covered with wqods, as they,-
prevent the accefs o f the fun's rays to,;fche earth, or to
the fnow which they may conceal, and prei nt more:
numerous evaporating furfaces, muff be colder thamopen
ppttnttiqs^-though fituated ufofche fame latituidfi»* AntU
.fince all-1 rafts o f land pijefept- infinite, varieties (of sfituf. -
% iq% Juniform refults daruiat here be expefted. Mrs.
KrawAN-pb^yqs .therefore, -that 'it is oh water Only
That .we’muft feck for ar ftandard-fi.tuatioh-With'whie&s
to compare the temperature o f •dfcbemfituationB.s i Now.
the globe contains, properly fpeaking, but two great
trafts o f «Water, -tire .Atlantic. Ocean, and the -Pacific
Ocean i which-' may each -bBi divided intoiro(iistb*vSM..
fouth, as they., lieon, the jborthern or fcuthewr 'fide o f
the equator. 'In. this" "trail of^vtafer, fie chofe that fitua-.
tion for a flandard which reco.mmends itf®imacffi-;by it s ,
fimplicity, and freedom from an yd; ut thrjiftull j ci manent'
caufes o f alteration o f temperature ; \;z;. that part o f
.the--Atlantic which lies' betvyeen cyp;'north and 45?
Touth.latittide, a nd~ex te n d i n g rdtithwards as far as the
G u lg ji .Stream, and to within a Few leagues o f the
Coaft o f America j—and thlt,par-t o f the ^aGific'.O.cean■
WhichJliee betWeen-459 north ajad 40 fol<th latitude,
and from 2cP to- etf£ilo}ig.itude._ Within this
Ipace, tlie mean annual temperature will be found as ex-
preffed -by the following table. ©EheifempeKftureb be-
yond 8o° latitude are added, though not ftriitly within 1
th e flandard,.
r N T R O D U C T I O N . lx ix
A . Tablé o f the mean Annual Temperature.-of . the flandard fituation, in every degree o f La titude..
f S m
■,Lat#/5j jT em p . . '. Lat. :- -Temp. 3
n g | .8 4 68,3 f e i'» / rr i y
'.S.. M l 63 h s i
; *}-,.«WÈÊÊÊm 1 I B
M W * S3,2 -r ■ 65.7 ‘ ; æî>‘4.„
? Wm IBM X lé û 'i Ùd3%X l,a
11 82,7 f -,i M i w m m m i
♦ÎÏO- f n | 3 0. 63 .168 -,pv# 1
11 M l » ‘1 62 37.8
12 Vu 41 ' 6 1 ,z iWJsK:
W m ? .+3 ?vjl- v
h 1 I aaSK M M W i
R: 80,4 ■i 4.4.1 58>4
iÆ îbW '.i 7^4'.'MWMMI
W mm g w « X M S I 1 | M M , 34.5 , 1 G 47 " 55 >6 M i
fïy F 't 78,3 ; M 1 :53>7' i M M l' ‘33.ÿ'
I M K ■ H k 78 ^a2'i ’ ■ 5° M w lM Em&< . Ü M ■ 'a • 7<5,5 -., 3 3 2 ,5
'~ 224l I ; 52 81 'St i ‘T s f i ’C -- - * 'Sÿ
: »5 : | : 54 ! K , : ’ i f o >7 j 2ff , 84 •. K
* 27 7 2 .8 ; M B v -47>j ., q I B 1 M H
S 2-8. 5 7 ... 4 ^ 7 iMS-fi i t r y
•29 v ' 7vri l 1 S i « # ' 87 > ■ aiiid...
.. 7 o»7v';4 . 88, . ;■ r .3 i , i b '1
, >a î î 1 69,9 60 H i i n i Æ S 4 # 'f 1
• 3* -’ .'og-jt ; J f; 61 ,1 ■ M 3 1 1
333. T h e rule by which tins table has been1 computed;
was .given by the famous aftrqnqmer-TbBiA s Mayer'
of Gottingen, and is as follows it was conftru&ediftomi
knowing the mean annual temperatuyes-of two latitudes.
Let s be the fine of the latitude; then the mean annual
temperature will he 843-53 Xr1*; that is, from 84 Ji®-
trail 5.3 multiplied into the fquare o f the fme o f the latitude,
and the remainder is the mean-annual temperature.
324. The temperatures off different years- differ; very-
little near the equator, but they differ moreiand more als
you approach the poles.-
325. I t fcarce. ever, freezes, in latitudes’; vundeil 359,v;
except in high fituations-.; and iC everyhails in latitudes
higher than 6'o°.
326- In latitudes betweemgj^ aiidj.6b‘’,' in. places adjacent
to the fea, it generally thaws When the fun’ s altitude^
is 40° or upwards ;; and feldom begins to freeze,.
untiLthe fun’ s meridian altitude is below 40°. 1 ,
3 2 7/ The greateft cold in a ll latitudes,^ our hemif-
jfliereyjs .generally about half an- hour- before fun-rife.-
T he greateft heat in all latitudes between 60° and '45? is
about half-paft 2 ' o'clock in the afternoon; between
latitudes 45° and 35°,iahout 2 o 'c lo c k ; between la,ti-
tudes 350 and 25", about half-paft 1 '.o’ clock ; and between
latitude,'25.? and the ; equator, about 1 o’ clock.
On fea, the difference between; the heat o f day and
nigbjiy is p&t'foVrea.t'as on- land, .particularly in low latitudes.—
. r
328- In all latitudes, January is the coldeft month.
July is th e warmeft month in all latitudes above 48 °;;
but in lower" latitudes,. Au guft'is the warmeft. The
temperature o f Ap r il' approaches more nearly to the-
mean annuid temperature, than any other month. ‘ ,
" 32;g . In theSigheft latitudes,; we often meet with au
heat iof 750- or-86°';. and in -latitudes ,:59° -and 6o° the
heat o f July is. frequently greater than in latitude 5 1°.-
33b. rA l l countries lying to the windward o f high'
mountains, or extenfive forefts, are warmer than thofe -
to the leeward in the fame latitude.
-331. T he .vicinity, to the fea is another circumftance
which •