ON THE KEW OBSERVATORY. 343 



A new Daniel Hygrometer, G, by Newman, has been used in various posi- 

 tions. 



The index correction of its immersed thermometer (found as above) was 

 — 0-3. 



The old Daniel Hygrometer,G, by Newman, which gave place to the above 

 (on February 1, 1851), had a small void in the mercurial column of the 

 immersed thermometer. 



The index corrections of this instrument, found as above, were at 32°— 0*5, 

 at 54°— O'S. 



The Saussure Hygrometer, H, of eight hours, made by Richer of Paris, in 

 1815, and described in the Journal de Physique, tom.xxxiv., p. 58, has a scale 

 of 100 parts. Some experiments made in September 1 850, seem to show that 

 the range of the index had extended 5 divisions on the side of " humidity," 

 and an equal quantity on the side of " dryness." 



An old Standard Thermometer made by Adams, in about 1768 probably, 

 for His Majesty George the Third's collection, is contained in a thick metallic 

 case, open in front. It has a brass Fahrenheit scale divided to 0'059 inch. 

 Its index corrections, when it was compared with C, as above, were at 

 32° — 0-5, at 54°— 0-5. 



A Regnault's Hygrometer and its Aspirator in their original forms have 

 been presented to us by Capt. Ludlow, R.E., together with several spare 

 glass cisterns whose lower parts are formed of thin black glass, in addition 

 to other cisterns compounded of silvered and glass tubes united by cement. 

 These instruments have been described by M. Regnault*. 



The comparisons of its two thermometers. No. 151 and No. 152, with 

 the Newman's standard C, on September 23rd, 1850, gave the following 

 results: — 



Newman's. No. 151. No. 152. 



32° 63°' 1 90°-9 



54-5 120-0 140-0 



72-5 166-0 178-6 



An improved Regnault's Hygrometer and Aspirator have been made, and 

 may be thus described : — 



A (fig. 1, Plate XX.) is the cistern, composed of a light and highly-polished 

 silver tube. A little cylinder, of solid glass, occupies about an inch, mea- 

 sured from its bottom, and aether the principal part of the space above the 

 glass. 



direct influences of the sun's rays, wind, and rain taken separately, and sometimes collectively, 

 by revolving A about B, but the difficult problem of always protecting them from all these 

 influences simultaneously, of also avoiding the anomalous effects of radiation and humidity 

 from neighbouring bodies, and of preserving a sufficiently free circulation of air, has, I fear, 

 still to be solved. 



* The distinguishing feature in the action of M. Regnault's admirable Hygrometer, as 

 compared with that of the Daniel, is the deportation of setherial vapour from the interior of 

 a mass of aether (as well as from the surface) contained in a light metallic or glass cistern, 

 by a current of air (bubbles) passed through the aether, and coohng the cistern down to, or 

 beyond, the dew-point. The aspirator, a vessel containing water, and whose upper part com- 

 municates with the cistern of the hygrometer (only) by means of a flexible pipe and stop- 

 cock, is employed to create (and regulate with great precision) the current of air (and con- 

 sequently the rate at which the cistern is cooled) by the flow of the water from its lower part. 



