3796] 
of the fubject, I am led to think that the 
thermometer invented by the late inge- 
nious Mr. $1x, and adapted to thow the 
greateft and leaft deyrees of heat in the 
obferver’s abfence, is an inftrument of 
lefs confequence to the meteorologift 
than may have been inagined. ‘The 
mean between the higheft and loweft de- 
gree of heat, obferved in any one day, 
may be very diftant from the true mean 
of the temperature of the whole of that 
day ; to this true mean we can only. 
make an approximation, near,.indeed, in 
proportion to the number of obferva- 
tions taken; but ftill it is only an ap- 
sroximation *. Philofophers, I appre- 
oy have not yet atcertaimed at what 
hour of the day, or at what diftance from 
fun-rifing, the heat is neareft the mean 
heat of the whole day. M. Dr Luc 
(Sur les Medifications de I Atmofphere, No. 
39s) infers, from obfervations taken 
every fifteen minutes, that the heat is 
nearest the mean heat of the whole 
day, when the fun has paffed through 
about one fifth part of his diurnal arch in | 
the heavens. I apprehend, the refult of 
all obfervations of this nature muft vary 
much, according to diferent fituations and 
expofures, to the prevalence of different 
winds in different climates, feafons, &c. 
3cc. From fome obfervations made every 
hour, from fun-rifing to fun-fet, on the 
varying temperature of the day, near the 
equinox juft now paffed, I find that the: 
- mean of the two obfervations at 8, A.M. 
and 2, P-M. approaches extremely near 
‘to the mean of all the hourly obferva- 
tions, from 6, A.M. to6, P.M. which, on 
the other hand, was found to be very 
diftant from the degree obferved at 8 h. 
24min. A.M. the time of the true mean, 
according to M. Dr Luc’s obfervations ; 
but I am fenfible that obfervations very 
often repeated, at different feafons, and 
in very different circumftances of wea- 
ther, &c. are requifite to afcertain this 
matter. 
I ufe two thermometers, both made 
by eminent artifts, and very exactly cor- 
refponding with each other : one, which 
I diftinguifh by calling it A, is placed in 

* Perhaps the beft method of afcertaining the 
true diurnal mean of heat is that recommended 
by Mr. Six, of finking a thermometer a few 
feet beneath the furface of the ground, in afhady 
fituation ; as the true annual mean has been 
thought to be moft accurately indicated by the 
_conftant temperature of deep wells and fprings. 
In this place, the fprings lic toa near the furface 
/ for this purpofe. 
Meteorological Obfervaiions at Chichefter. 
367 
a box open at bottom, at the diftance of 
about an inch and a half trom the wail, 
en the outfide of a window, about iix- 
teen feet and a halt trom the ground < 
it has a north-weli expolure,- and is come 
pletely in the ihade tili long after 
2, P.M. but though not at ail aiteéted 
by the direét rays of tae fun, Ll have cer- 
tain reafon to believe that this initrument 
is influenced by its reficcted light and 
heat from fome buildings extending at 
the diftance of thirty or forty teet to the 
north and north-eait. The other ther- 
mometer, which I call 4, 1s placed about 
four feet anda halt from the ground, at the 
‘northern entrance of a low arched way in 
the garden. It is perfectly {kreened from 
the influence either of direct or reflecied 
light, yet the air has tree accefs to 
it. The difference of the temperature 
denoted by thefe thermometers 1s fome- 
times very confiderable-. In a warm and 
clear fummer day, with an eafterly wind, 
A fhall be fometimes five, fix, or even 
feven degrees above Bat 2,P.M. On 
the contrary, a cloudy iky and a wefterly 
wind will bring them much nearer to 
each other at that hour. In the fummer 
and autumn, at 8, A.M. :A is ufually half 
a degree, or one degree, higher than B, 
and at 2, P.M. it is trom two'te fix de- 
grees higher than B. In winter and 
ipring, at 8, A.M. Bis ufually from half 
a degree toa degree and halt higher than 
A; and at 2, P.M. from half a degrce ta 
two degrees lowerthan A. Very rarely, 
within thefe two years, has B been higher 
than A, at 2, P.M. though feveral tumes 
it has pointed at exactly the fame degree 
at that hour. Any very confiderable de- 
viation from thefe relative heights of A 
and B, at the hours mentioned, has been 
ufually followed by a proportional change 
in the temperature of the air, and very 
frequently by change of weather. The 
difference of the monthly mean of the 
two thermometers, when greateft (which. - 
is always in the hotteft months) has 
been 2°05; when Jeaft, it has been 
0°.27-+. In the tables, the mean is that 
of B; the extreines thofe of A. 
t is proper to remark, that the con- 
fiderable difference obfervable in the ftate 
of the hygrometer for the two laf years, 
muft, in part, be attributed-to this cir- 
cumttance ; that till the beginning of 
March 1795, this inftrument was kept 
within doors, in a paflage, at a great 
diftance from any fire-place, and near a 
window which was frequently open : ia 
this fituation, however, it did not truly 
imdicate the degree of moifture of the 
I, external 
¥ 
