MR. HUDSON’S HOURLY OBSERVATIONS ON THE BAROMETER. 591 
of altitude in the station of observation upon the variations. 6. A complete 
examination of the effect of temperature in influencing the changes of the baro- 
meter. 7- Whether, after the application of the ordinary corrections, the 
changes in the length of the mercurial column correspond accurately with 
those which take place in its absolute weight. 
With regard to the first of these inquiries, Mr. Dollond has, at his own ex- 
pense, furnished me with an instrument exhibiting the changes of atmospheric 
pressure without involving the agency of the mercurial vapour; and with 
which I propose to make a series of observations, simultaneously with the 
Standard Barometer. It is a Baroscope of considerable dimensions, and the 
same in principle as the well-known instrument of Boyle, having a thin 
glass globe, of one foot and a quarter in diameter, counterpoised by a solid 
sphere of lead. From an abstract of a memoir by Signor Avogadro, contained 
in the fourth number of the Annales de Chimie for the present year, on the 
elastic force of mercurial vapour at different temperatures, it appears that the 
effect of this vapour in the vacuum chamber of a mercurial barometer would 
not be sensible at the ordinary temperatures of the atmosphere, as its tension 
at 212° F. appears to be equal to only -001 inch of mercury ; and Dr. Prout 
has allowed me to state, that from his own investigations it appears to have no 
influence under common circumstances, he having, in summer when the tem- 
perature was unusually high, cooled down a mercurial barometer, by means 
of the evaporation of ether, to 32°, without detecting any such influence, 
after the requisite correction for the temperature of the mercury itself had been 
applied. Mr. Snow Harris of Plymouth, having made a variety of experi- 
ments on the effects produced on barometers by the introduction of different 
gases into their vacuum chambers, has kindly offered to furnish me with the 
detail and results of his experiments, to lay before the Society in connexion 
with my own. — With regard to the second inquiry, I have compared two ex- 
cellent and similar mountain barometers, for the use of which I was indebted 
to Mr. Cary, — first, under the same circumstances and temperature; and after- 
wards under the same circumstances in every respect excepting the tempera- 
ture, which in the latter case was considerably raised. The mean difference 
obtained in one case was not sufficiently unequal to that obtained in the 
other to indicate any error or discrepancy in the Tables by which the ob- 
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