oi' manna ; it then exuiles spontaneously ; 
hut to obtain it more copiously, incisions 
are made through the bark by means of a 
sharp crooked instrument ; and the season 
for performing this is in the middle of the 
summer. 
MANOMETER, or ManosCope, an in- 
strument to show or measure the alterations 
in the rarity or density oi the air. The ma- 
nometer ditfers from the barometer in this, 
that the lattei only serves to measure the 
weight of the atmosphere, or of the column 
of air over it : but the former, the density 
of the air on which it is found; which den- 
sity depends not only on the weight of tlie 
atmosphere, but also on the action of heat 
and cold, &c. Authors, however, generally 
confound the two together; and Mr. Boyle 
himself gives us a very good manometer o^f 
his contrivance, under the name of a stati- 
cal barometer, consisting of a bubble of 
thin glass, about the size of an orange, 
which, being counterpoised when the air 
was in a mean slate of density, by means of 
a nice pair of scales, sunk when the atmo- 
sphere became lighter, and rose as it grew 
heavier. Other kinds of manometers were 
made use of by Colonel Roy, in his at- 
tempts to correct the errors of the barome- 
ter. “They were,” says he, “of various 
lengths, from f )ur to upwards of eight feet : 
they consisted of straight lubes, whose bores 
were commonly from one-fifteenth to one- 
twenty-fifth of an inch in diameter. The 
capacity of the tube was carefully mea- 
sured, by making a column of quicksilver, 
about three or four inches in length, move 
along it from one end to the other. These 
spaces were severally marked, with a fine- 
edged file, on the tubes; and transferred 
from them to long slips of pasteboard, for 
the subsequent construction of the scales 
respectively belonging to each. The bulb, 
attached to one end of the manometer at 
the glass-house, was of the form of a pear, 
wliose point being occasionally opened, dry 
or moist air could be readily admitted, and 
the bulb sealed again, without any sensible 
alteration in its capacity. The air was 
confined by means of a column of quick sil- 
ver, long or short, and with the, bulb down- 
ward or upwards, according to the nature 
of the proposed experiment. Here it must 
be observed, that, from the adhesion of the 
quicksilver to the tube, the instrument will 
not act truly, except it be in a vertical po- 
sition ; and even then it is necessary to give 
it a small degree of motion, to bring the 
quicksilver into its true place, where it will 
MANOMETER. 
remain in equilibrio, between the exteriof 
pressure of the atmosphere on one side, and 
the interior elastic force of the confined ait 
on the other. Pounded ice and water 
were used to fix a freezing point on the 
tube ; and by means of salt and ice, the aif 
was further condensed, generally four, and 
sometimes five or six degrees below zero. 
The thermometer and manometer were 
then placed in a tin vessel among water, 
which was brought into violent ebullition ; 
where, having remained a sufficient time, 
and motion being given to the manometer, 
a boiling point was marked thereon. After 
this the fire vVas removed, and the gradual 
descents of the piece of quicksilver, corres- 
ponding to every twenty degrees of tempe- 
rature in the thermometer, were succes- 
sively marked on a deal rod applied to the 
manometer. It is to be observed, that 
both instruments, while in the water, were 
in circumstances perfectly similar ; that is 
to say, the ball and bulb were at the bot- 
tom oi the vessel. In order to be certain 
that no air had escaped by tlie side of the 
quicksilver during the operation, the mano- 
meter was frequently placed a second time 
in melting ice. If the barometer had not 
altered between the beginning and end of 
the experiment, the quicksilver always be- 
came stationary at or near the first mark. 
If any sudden change had taken place in 
the weight of the atmosphere during that 
interval, the same was noted, and allow- 
ance made for it in afterwards proportion- 
ing the spaces. Long tubes, with bores 
truly cylindrical, or of any uniform figure, 
are scarcely ever met with. Such, how- 
ever, as were used in these experiments, 
generally tapered in a pretty regular man- 
ner from one end to the other. When the 
bulb was downwards, and the tube narrow- 
ed that way, the column of quicksilver con- 
fining the air, lengthened in the lower half 
of the scale, and augmented the pressure 
above the mean. In the upper half, the 
column being shortened, the pressure was 
diminished below the mean. In this case, 
the observed spaces both ways from tlie 
centre were diminished in the inverse ratio 
of the heights of the barometer at each 
space, compared with its mean height. If 
the bore widened towards the bulb when 
downwards, the observed spaces, each way 
from the centre, were augmented in the 
same inverse ratio ; but in the experiments 
on air less dense than the atmosphere, the 
bulb being upwards, the same equation was 
applied with contrary signs; and if any ex- 
