Description of a Self-registering Hygrometer. 247 



but should the process of evaporation diminish, the liquid uill 

 again move towards D, while the inde:; is left behind, thus markr 

 ing the maximum of dryvess in the absence of the observer. 



To find the greatest degree of moisture, another instrument is 

 to be emfj'nved, wliich is represented in fig. 2. The oidy dit- 

 ferenfe between this and the former is, that the air in the two 

 bulbs is to be so adjusted, that when thev are at the same tem- 

 perature, the liquid mav stand near the extremity C, the di- 

 staiK-e between C and zero being a little more than the length 

 of the index a ; and the bulb B is to be covered as A was in the 

 former. The scale is graduated as before. When the instru- 

 ment is adjusted and exposed, evajjoration goes on from the sur- 

 face of B; and the air within being thereby contracted, the li- 

 quid moves towards D, continuing to do so till the maximum 

 effect is produced. When the evaporation diminishes, the liquid 

 is again force-l backwards towards C, till it arri^'es at the index 

 a ; and should the evaporating force still continue to diminish, 

 the index itself is then carried towards zero, till the evaporation 

 be at its ininiuiuni. The liquid then becomes stationary; aiid 

 though it should afterwards mount higher, in consequence of an 

 increased evaporation, still the index remains at the loweist point 

 to which the liquid had sunk, thus marking the minimum of 

 dryness in the absence of the observer. 



Your readers will observe, that in the modification which I 

 have now proposed of the original hygrometer of Professor Leslie, 

 no new principle has been introduced; the contrivance for 

 markinff the extremes being the same as that which was invented 

 by Dr. Rutherford, and which has been long employed in con- 

 structing a minimum thermometer. I can lay no claim, there- 

 fore, to the honour of a discovery, but 1 hope I have some little 

 to that of an improvement. It is well known to meteorologists, 

 that observations of temperature were comparatively of little use 

 till the invention of self- registering thermometers; and why may 

 we not be allowed to hope, that the use of a self-registering 

 hygrometer will hereafter bring to light some important facts 

 regarding the laws which regulate the distribution of moisture in 

 the atmosphere ? 



One word as to the construction of the instrument, and I have 

 done. In the fir^t, and indeed the only attempt that I have vet 



Q4 hud 



