*§^ •Mi'^TJ Hopkins on the Diurnal Changes 



the variiible atmospheric pressure, and the semi-diurnal fluc- 

 tuations of the barometer. The vapour is, at a certain hour 

 in the morning, at its minimum quantity, from which it in- 

 creases during the day up to its maximum ; after that it de- 

 cHnes, and its variable pressure is exerted on the mercury of 

 the barometer, and affects the height of the coUimn. This 

 takes place in a less or greater degree in all latitudes, though 

 to the greatest extent near the equator. 



The quantity of vapour existing in the atmosphere in each 

 hour of the day is ascertained from the dew-point, or point of 

 condensation; it having been found that each particular 

 quantity of vapour diffused through the air has its separate 

 dew-point. The dew-point is therefore taken as the measure 

 of the quantity of aqueous matter existing in the atmosphere, 

 and of the vapour pressure, at every period of time. This 

 pressure, thus ascertained, being deducted from the whole at- 

 mospheric pressure, furnishes the amount of the gaseous' 

 pressure, as given in our Meteorological Registers and Tables. 



But, is the dew-point a correct measure of the quantity of 

 aqueous matter that passes into and remains in the atmosphere 

 during the different times of the day ? On the answer to this 

 question it depends whether the hourly vapour and gaseous 

 pressures on the barometer are, or are not, correctly given in 

 our registers. If the dew-point be a true measure, then the 

 pressure arising from aqueous matter may be taken to be such 

 as is stated in those registers, and so far all the reasonings 

 respecting the causes of the diurnal fluctuations of the baro- 

 meter may be correct ; but if the dew-point is a fallacious 

 measure of that pressure, then the alleged facts may be un-^ 

 founded, and the conclusions drawn from them erroneou'si^^^; 



There is reason to believe that in certain parts of the world, 

 and for considerable periods of time, the dew-point may be a 

 correct indicator of the pressure of aqueous matter, but in 

 other parts it may not ; and in order that we may trace this 

 difference, in different times and places, we will inquire what 

 are the relative quantities of vapour that hourly pass into the 

 atmosphere, in some of those parts from which we have been 

 furnished with accounts, and endeavour to learn whether 

 those quantities are such as to accord with the dew-points. 



Kaemtz, a German meteorologist, in his Course of Me- 

 teorology, has furnished tables of the hourly vapour pressure 

 in different places, deduced, in the usual way, from the dew- 

 point, and among them of that which is found to be the mean 

 of the year at Appenrade, in Denmark, from seven in the 

 morning to eleven in the evening. At seven, the pressure, in 

 French measure, is 8 millimetres '119, from which it increases 



