4fS^ Mr«\'T. Hopkins on the Diurnal Changes 



between the two thermometers is <^reater than it was at elevenc 

 o'clock, two hours before the highest temperature ! Evapo- 

 ration must therefore have been more energetic, and must 

 have continued to throw into the atmosphere more vapour 

 from eleven to three than it had done four hours earlier ! 

 Now, if increase of vapour pressure always accompanied in-^ 

 crease of vapour, the increase of pressure at Plymouth must 

 have continued up to three o'clock ! If however we look at 

 the curve or line of the dew-point, which represents vapour 

 pressure in the diagram, we find that it did not rise after 

 eleven o'clock, but continued stationary from that hour until 

 4' P.M. ! It is therefore apparent, that at Plymouth the quan- 

 tity of vapour which by evaporation passed into the atmo- 

 sphere in the middle of the day, to add to the general atmo- 

 spheric pressure, in some form, was not indicated by the dew- 

 point. And analogy authorises us to infer, that in other parts 

 of the world, the state of the dew-point during the same 

 portion of the day does not express the quantity of vapour 

 that has passed into the atmosphere, and which must have 

 added to its general pressure on the barometer. 



In the Toronto registers, reported to the British Associa- 

 tion at York in 1844 by Col. Sabine*, the state of the wet-bulb 

 thermometer is not given. But we may assume that if it had 

 been given, it would have shown the same features as those 

 we have in the Plymouth registers and diagrams. In this 

 report it is, however, stated that Mr. Caldecott has trans- 

 mitted to England five years of hourly observations with the 

 wet and dry-bulb thermometers at Trevandrum, near Cape 

 Comorin, where a large quantity of vapour generally exists 

 in the atmosphere. It appears from these accounts that the 

 minimum and maximum pressures of the atmospheric vapour 

 are there found to occur within three hours of each other, — 

 the minimum coinciding with the coldest hour, 6 a.m., and 

 the maximum occurring so early as at nine in the forenoon ! 

 Now, it is very desirable that it should be ascertained whether 

 evaporation did or did not go on freely from the wet-bulb 

 thermometer from six in the morning, not only until nine in 

 the morning, but until two in the afternoon, the time of the 

 highest temperature. Although the dew-point ceased to rise 

 at nine, it is to be presumed, reasoning from analogy, that, 

 energetic evaporation continued through the middle of thaj 

 day, and it probably was (as at Plymouth) more active be- 

 tween nine and two in the day, than it had been in any part 

 of the time between six and nine in the morning. And the 

 vapour which was thus produced at Trevandrum between 



* This report was inserted jp the February Number of this Journal for' 

 the present year.— Ed, 



