THERMOMETER AND PYROMETER. 



strument, held in the vertical position, 

 cools, the globule, will follow the thread 

 into the stem. 



This instrument is of great delicacy ; 

 being capable of showing a difference 

 of altitude of not more than three feet ; 

 but, unfortunately, although not very 

 bulky, it is not very portable, from the 

 liability of the stem to be broken by the 

 weight of the bulb, even from the usual 

 jolting of a carriage ; an accident which 

 happened thrice to the writer of this, 

 within one month. From this cir- 

 cumstance, and its price, it is not likely 

 to supersede the barometer in geological 

 surveys. 



4. M. Le Roi was the first who sug- 

 gested the temperature at which dew 

 begins to be deposited as a method of 

 ascertaining the moisture of the air. 

 De Luc has the merit of having proved 

 that the quantity and force of vapour in 

 a vacuum of any given dimensions, are 

 equal to its force and quantity, in an 

 equal volume of air, at the same tem- 

 perature ; or that the force and quantity 

 of vapour in the air are dependant on 

 its temperature.* This was confirmed 

 by Mr. J. Dalton, t who investigated 

 the force of vapour, at every tempera- 

 ture, from to above 212 Fahren- 

 heit, and expressed this force by the 

 height of the mercurial column, which 

 it could support in a Torricellian 

 tube. These results are given in a ta- 

 bular form, and are thus easily applied 

 to hygrometric purposes. Dalton finds 

 the dew point, like Le Roi, by pouring 

 cold water into a glass, and marking the 

 temperature at which it just ceases to 

 cause the deposition of dew on the sides 

 of the glass, in the open air. This is 

 the point at which, in an air of that 

 temperature, dew would just begin to be 

 formed. From this fact he is able to 

 infer, not only the force exerted by the 

 vapour, but its quantity in a perpendi- 

 cular column of the whole atmosphere, 

 and the force of evaporation at the time 

 of observation. 



Thus, if the dew point be 45, the 

 force of vapour in Dalton's table = 0.3 16 

 of an inch of the mercurial column, or 

 the one-ninety fifth of the whole atmos- 

 pheric pressure ; or, if the specific gra- 

 vity of steam be 0.70, the weight of 

 the steam or vapour in a given volume 

 of air will be the one hundred and thirty- 

 sixth part of the whole. Now, as the force 

 of a whole atmosphere of steam, at the 



* Recherches sur ]es Modifications de 1'Atmosphere. 

 t Manchester Memoirs, vol. v. 535. vol. i. new 

 series, p. 252. 



49 



surface of the earth," would be the weight 

 of a perpendicular column of it, and as 

 in a mixed atmosphere of steam and air, 

 the force exerted by each is as their re- 

 lative weights, it follows, that when the 

 dew point is 4 5, the whole superincum- 

 bent column of vapour in the atmosphere, 

 being equal to the one-ninety-fifth of 

 the whole atmospheric pressure, will be 

 equivalent to a pressure of 4.30 inches 

 of water ; or the vapour, if condensed, 

 would afford that depth of water. From 

 these data, Dalton has shown how we 

 can find the force of evaporation at a 

 given time : for the quantity of water 

 evaporated from a given surface is 

 proportional to the maximum force of 

 vapour at the temperature of that sur- 

 face ; it being understood that the va- 

 pour is still in contact with a surface of 

 water. Hence, if we have the dew point 

 45, while the temperature of the air 

 is 50, by subtracting the force of va- 

 pour at 45 from that of 50, we shall 

 obtain the force of the evaporation at 

 that time thus, .375 .316=.059, the 

 force of evaporation. 



5. It is on this principle that DanielZ's 

 Hygrometer is constructed ; the inven- 

 tion of a gentleman distinguished for 

 his meritorious labours in meteorology. 

 It was published in 1820, along with a 

 meteorological table, and seems to have 

 been suggested by the cryophorus of 

 Wollaston. J The form of the instru- 

 ment is seen, as last improved by Mr. 

 Daniell, in (fig. 48.) The ball a is of 



Fig. 48. 



t Quarterly Journal of Science, vol. viii. 299 see 

 also vol. ix. &c, 



