90 REPORT—1840. 
150. There is no instrumental result to be received at all times 
with more doubt than the absolute height of the barometer. I 
have had occasion to compare many which have the character 
of being standard instruments, and have found the most serious 
inconsistencies. There are few points at which the mean press- 
ure of the atmosphere can be said to be accurately known. 
Even at Paris there is some little doubt* ; when reduced to 
the level of the sea, itappears to be 760°85 millimetres}. From 
seventeen years of very careful observations at Marseilles, it is 
761°61 at 0°, and at the level of the seat, clearly indicating 
that Paris lies considerably to the N. of the maximum pressure 
zone. It may be doubted whether we possess in this country 
any satisfactory evidence of the mean height of the barometer 
at the level of the sea$. 
C. Barometric Oscillations ||. 
151. The investigation, by Mr. Snow Harris, of the diurnal 
atmospheric tide by means of hourly observations, continued at 
Plymouth night and day for three years ], has led to a very 
satisfactory determination both of the epoch and amount. Ac- 
cording to him, the maxima occur at 93 4.m., and at 10 p.m., 
the minima at 445 a.m., and 31" p.m.; and the measures ap- 
pear to be the following (approximately) :— 
Rise from 4 a.m. to 10a.M. . . *014 inch 
al), 20 4.m. to "Ss Pom. >.” oS! "OL ger 
Rise’ 5, 3°P.M.tolO p.m. . . “O21 ';; 
Fall ’,, -10' p.m. to 4am...) . *O18~',, 
The maximum oscillation here appears to be between 3 P.M. 
and 10 p.M., and amounts to 0°53 millimetres. My formula** 
gives 0°60. Mr. Harris deduces 29°800 for the mean pressure 
60 feet above the level of the sea, which agrees nearly with the 
observations at Somerset House; but there is some material 
discrepancy in the observations of mean height for different 
years. 
* M. Bouvard gives 755°99 mm. reduced to 0° c. from eleven years’ obser- 
vation at Paris; M. Arago, 755°43 mm. from nine years’ observation. 
+ Arago. t Kindly communicated to me by M. Valz, of Marseilles. 
§ The Royal Society Observations since Nov. 1837 (see Phil. Mag., xii. 
204), may one day afford this, but the period is as yet rather too short. 
| Last Report, p. 229. _Mahlmann, p. 89. 
4 The results were partly communicated to the British Association in 1839 
(see Atheneum, 14th Sept.). The following results, which are corrected for — 
temperature and embrace three years, were communicated to me by Mr. Harris 
himself. The agreement of the three years is very satistactory, so far as the 
form of the curves is concerned. See also British Association, Eighth Report, 
. 22. 
** Edinburgh Transactions, vol. xii. 
