On the Comet of October 1st, 1847. 83 
It appears from the rule as stated above, for finding the values 
a of D and P, that the quantity d, which expresses the density of 
an the fluid in the receiving vessel, will not enter at all into the for- 
mula which expresses the velocity of the flow, nor into that 
which expresses the quantity discharged in a given time, pro- 
vided it be equal to or less than half the density in the discharg~ 
ing vessel. Hence it follows that the fluid in the receiving ves~ 
sel, if its density does not exceed half the density in the dis- 
charging vessel, will have no effect whatever upon the flow. 
Consequently air or steam will rush into a vacuum no faster than 
into a vessel containing fluid of half its density. On the con- 
trary both the velocity of the flow and the quantity discharged 
in a given time will be the same in both cases; and so also if the 
density in the receiving vessel is any quantity dess than half the 
density in the discharging vessel, the flow will be the same in 
: velocity, quantity, and density, as into a vacuum. Accordingly a 
Vessel containing steam of a density due to a pressure of ten or 
2 any other number of atmospheres, will empty itself no faster 
into a vacuum than into the open air, until in the progress of the 
discharge, the density is reduced below that due to the pressure 
of two atmospheres. It will be readily seen that these conclu- 
‘es clusions have an important bearing upon the construction of the 
oe Steam engine. 
Art. XIII—On the Comet of 10th month, (October Ist,) 1847 ; 
“e by Wituram Mircuett, of Nantucket. 
Tats interesting body, as it has been denominated by Prof. 
Peirce, was first discovered by Maria Mitchell of Nantucket. As 
arelaxation from the severer toil of a systematic course of peri- 
odical observations, she had employed the intervals through the 
last year in sweeping for comets; but her labors had hitherto 
been only rewarded by a familiarity with comet-resembling neb- 
ule, which she had constantly and carefully recorded. The in- 
Strument employed on these occasions was a forty-six inch re- 
fractor with three inches of aperture, mounted on a tripod, and 
ished with a terrestrial eye-piece of moderate power. 
above Polaris (then nearly on the meridian) and nearly corres- 
ponding with it in right ascension. There was searcely a doubt 
of the cometary character of this object, inasmuch as the region 
Which it occupied, had often fallen under the serutiny of the ob- 
server. Still, as the object was faint and the weather uncom- 
Monty clear, a possibility existed that this too was a nebula not 
