46 



REPORT — 1859. 



With reference to the second hypothesis, that of overflowing currents, it was stated 

 that not only were there no grounds for it, but there were several facts quite opposed 

 it. Among others, it was pointed out that the best marked of the semidiurnal varia- 

 tion of the barometer at Makerstoun in 1843-46, was that for the night (9 p.m. to 

 9 a.m.) during the winter quarter, when the tension of vapour and temperature of 

 the air were nearly constant. 



Mr. Broun now adduced the results of a series of observations made under his 

 direction, by fifteen observers, at Trevandrum, at the base of the Ghats, twenty miles 

 distant ; and at three other stations, rising successively on the sides of the Agustier 

 Mallay, by 1500 to 1700 feet, the highest being the Peak Observatory, 6200 feet above 

 the sea-level. These series show completely the insufficiency of all the usual hypo- 

 theses. 



The following Table contains the mean barometer oscillations derived from a 

 month's hourly observations in the commencement of 1859 : — 



It appeared from these observations that the night oscillations (that between 9 p.m. 

 and 9 a.m.) had nearly the same value at Trevandrum three miles from the sea, and 

 at different heights on the Ghats twenty miles from Trevandrum ; the oscillation 

 being on the whole greatest at the highest station. The day oscillation diminishes 

 as we ascend, the diminution being partly due to the expansion of the atmosphere 

 during the day, by which part of it previously below the upper stations is carried 

 above them. 



It seemed probable that the oscillations were chiefly due to an action upon the 

 upper or dry atmosphere, indicating, the author conceived, an electrical or magnetical 

 result. 



Mr. Broun proposed in 1857 a theory of the diurnal variations of the barometer, 

 which agreed to some extent with one communicated to him by Dr. Lamont in a 

 letter (dated June 4, 1859), but published elsewhere by Dr. Lamont. 



Dr. Lamont's hypothesis was founded on the electrical action of the sun, whereas 

 Mr. Broun's, as at first proposed by him, was founded on the sun's magnetical 

 action ; he proposed in the present state of the facts to place both hypotheses under 

 the following general form. 



The sun by its electrical action (static or dynamic) on our atmosphere and the 

 earth gives to the atmosphere an ellipsoidal form with the longer axis nearly under 

 the sun ; this ellipsoid, following the sun, produces the semidiurnal oscillation of the 

 barometer, the extent of this electrical action probably depending on the relative 

 dryness of the air. 



In Dr. Lamont's view, the action is electrical induction on the atmosphere (the 

 sun's electricity supposed positive) ; in Mr. Broun's view, magnetical induction on 

 the atmosphere and earth ; by both the part next the sun is attracted, and on the 

 opposite side repelled, or vice versa. 



The author now referred to the annual variation of the barometer, and concluded — 



That the annual variation within the tropics was quite a local result, the range of 

 the monthly mean pressures being about one-tenth of an inch for each four or five 

 degrees Fahrenheit of range of monthly mean temperature. 



That the mean pressure of the whole atmosphere is greatest in December or 

 January, and least in June or July ; this difference being more marked when the 

 computed dry air pressures are considered. 



Finally, the author remarked that if the sun's action on our atmosphere resembled 

 its action on the gases of comets, the pressure should become greater as the earth 

 approached the sun, that is in December ; but it was believed that the difference 

 found between the pressures in December and June, was chiefly due to the greater 



