Meteorologic\l Observations. 123 



The annual normal curve of relative humidity is given by the 

 f'^rmula : 



H = h+i8-]j8 sin (ni50 + 470-5) 

 + 4'87o sin {n^o° + 24^°'^) 

 + i'i8o sin (n45° -f i97°"2) 

 + i"569 sin (n6o° + 44°'i) 

 + 



In the same way as before, the second component vanishes 

 foi the epoch ^h. 40m., gh. 40m., a.m. and p.m. And at these times 

 the value of the fourth component is -1.5% very nearly. The mean 

 humidity for either pair of hours, therefore, will fall short of the 

 annual mean by about one and a half per cent. 



The term of third order vanishes at ^h. 37m., 7h. 37m., iih. 

 37m., a.m. and p.m. Whence we may consider the observations 

 made at (3h. 30m., iih. 30m., i9h. 30m.) or (7h. 30m., i5h. 30m., 

 23h. 30m.) as giving the mean humidity of the year. 



The fourth term vanishes at 2h. i6m., 5h. 16m.. 8h. i6m., 

 iih. i6m., a.m. and p.m.; and, therefore, the observations of dry 

 and wet bulb at (II., VIII., XIV., XX.) or (V., XL, XVII., XXIII.) 

 will give the mean annual relative humidity with reasonable accuracy. 



The epochs of humidity agree so closely with those of tem- 

 perature, in each harmonic term, because the humidity curve is 

 practically an inverted curve of temperature. The angular differ- 

 ences would, indeed, be exactly 180°, in each case, if the quantity 

 of moisture in the air were constant, the departure from this angle 

 arising out of the diurnal variation in the dew-point curve. The 

 clear relationship between the corresponding harmonic terms of 

 barometric pressure and dew-point has been mentioned elsewhere.* 



We may here conveniently summarise the best hours of obser- 

 vation indicated by theory : 



1. For a single observation per diem, XX. ; the temperature 

 being combined with the maximum and minimum of the day, and 

 the pressure increased bv 'or inch — unless the barometer be read 

 at XXI. 



2. For two observations at homonymous hours : 



Pressure I.. Xlli., or VII.. XIX. 



Temperature IV., XVI.. or X., XXII. 



Dew-point I., XIII., or VII., XIX. 



Humidity IV., XVI., or X., XXII. 



3. For three observations at intervals of eight hours : 



Pressure O, VIII., XVI., or IV., 



XII.. XX. 

 Temperature J3h. 30m.. 7h. 30m.. 1 5h. 



30m.: or 3h. 30m., rih. 30m.. iph. 30m. 

 Dew-point O, VIII., XVI., or IV.. XII., 



XX. 

 Humidity 23h. 30m.. 7h. 30m.. i^h. 



30m., or 3h. 30m., iih. 30m., i9h. 30m. 



*" Elementary Svnopsis. &c." Trans, of the S.A. Phil. Soc. Vol. XIV. Part :. 



K 2 



