£13 A TREATISE 



eases (a), I have the satisfaction to and that the 

 very same tendencies have been observed to prevail 

 on the opposite side of the globe. We may there- 

 fore now venture to allow them a more extensive 

 range ; and it will, no doubt, be considered of some 

 importance to establish in certain latitudes (b) the 

 existence of a law in nature by which the Mercury 

 of the Barometer, let the standing weight and pres- 

 sure of the atmosphere be what it may, is liable to 

 the effects of a constant and regular periodical diur- 

 nal fluctuation; for it will then follow that the power 

 of each succeeding hour to raise or sink it, is liable 

 to differ from that which went before; that the height 

 of the Mercury, therefore, taken only at two or three 

 stated hours of the day, cannot with propriety be as- 



(a) The Note referred to in Dr. Moseley's Treatise is this: — 

 44 It has been observed in these and more equatorial regions, that 

 44 though the barometer is useless in indicating the variations of the 

 44 weather, it exhibits a phenomenon not correctly ascertained ifi 

 44 temperate climates; which is that the Mercury has two diurnal 

 44 motions of ascent and descent, of nearly a line corresponding 

 44 with the course of the sun ; ascending as the sun approaches the 

 44 zenith and nadir, and descending as the sun deviates from these 

 44 points. It remains stationary at its lowest and highest degrees 

 44 for some hours." 



(b) As far as I can judge from the following extract from Father 

 Cotte's Memoir on the prevailing winds, &c. &c. which I have 

 just met with in the Edinburgh Magazine for March 1792, there 

 seems to be great reason to beiieve, that similar fluctuations take 

 place in the Mercury, in the different latitudes of Europe ; and 

 that they are not entirely confined to the regions under the equator. 



4t The Mercury is generally a little lower about two o'clock ia 

 44 the afternoon than at any other time of the day ; and it is high- 

 44 est towards eight o'clock at night. I would compare this fact 

 44 without pretending to draw any consequences from it, with the 

 u phenomenon of the magnetic needle, the greatest variation of 

 •' which from north towards west takes place about two or three 

 44 in the afternoon, and the least about eight o'clock in the morn- 

 44 ing."~ Vide the Edinburgh Magazine for Ma) ch IJQ2) page 21 1 . 

 par. 6. 



sumerl 



