Barometric Pressure. 19 



monthly values of mi and m^, on the other hand, are twice as great in 

 the summer as they are in the winter. Also the mean monthly 

 differences between any two other given phases vary considerably 

 month by month. 



The fact that there is some sort of diurnal oscillation of the 

 barometer has probably been known for more than two centuries ; but 

 it is only within the last century that any really scientific effort has 

 been made to understand it. In 1666 Dr. Beale, an old Cambridge 

 man, observed that generally, in settled and fair weather, the mercury 

 stands higher than it does during rain or storm ; and often, both in 

 winter and summer, is higher in the colder mornings and evenings 

 than in the warmer mid-day. No great advance upon this seems to 

 have been made for many years (excepting that the fact of the diurnal 

 oscillation impressed itself gradually upon observers), chiefly, 

 jierhaps, because of the imperfections of barometers in those days. 

 For it was not until 1738 that Orme invented his method of thoroughly 

 boiling the mercury in the barometer tubes. Indeed, so little attention 

 was paid to the phenomenon that there is not a single paper devoted 

 to it in the Philosophical Trajisaciioiis, at any rate, down to the 

 beginning of the 19th century. Dalton, in the first edition of his 

 Meteorological Observations and Essays (1793), does not so much 

 as allude to it, though he deals at some length with barometric 

 variations from day to day. In his second edition (1834) he mentions 

 it, without apparently suspecting that the oscillation is semi-diurnal, 

 and ventures upon an explanation of the cause of it. Even Harvey, 

 in his otherwise excellent Meteorology (1845), w^astes no words over 

 it at all. Meanwhile Hudson (1832) had made a series of hourly 

 observations at the rooms of the Royal Society, and determined the 

 mean diurnal curve with a considerable degree of exactitude. It was, 

 however, only within the last 50 years that the matter was seriously 

 taken up, during which time a number of theories of undoubted merit 

 have been propounded. One or two of these I propose to mention as 

 illustrating the progress of meteorological methods. 



Here is Sir John Herschel's account of the barometric oscillation 

 as it was regarded in his day : — 



" (165.) The periodic fluctuations of the barometer are 

 annual and diurnal. The consideration of the former will 

 enable us to form a neater conception of the mode in which the 

 latter arise. When it is summer in one hemisphere it is winter 

 in the other. Hence the air generally incumbent on the heated 

 hemisphere is dilated, and expands both upwards and laterally 

 not only by its own increased elasticity, but also by the increased 

 production of vapour. It therefore not only encroaches on the 

 other hemisphere by lateral extension, but, what is far more 

 influential, flows over upon it. In order to perceive clearly the 

 nature of the process, we must separate in idea the aqueous and 

 aerial constituents of the portion of the atmosphere so 

 transferred. The generation of the former goes on in the heated 

 hemisphere, and replaces, in part at least, the loss of pressure 

 arising from the transfer of air, while in the other the excess 



