4 



130 PHILOSOPHICAL TRANSACTIONS. [aNNO 1771. 



but in this case, the adjustment must be repeated in the same manner, by drying 

 and wetting as before-described. 



It is to be remarked that, as the cord is supposed impregnated in a given de- 

 gree with common salt, and this not liable to evaporate, care must be taken in 

 wetting, that no drops of wet be suffered to fall from the cord, to preserve the 

 original quantity in the cord. 



It appears from many observations, that in the compass of 1 1 months, the 

 cords had stretched the value of 5°: and he also observed that they both had 

 contracted their compass about 10°. Mr. S. would therefore recommend, that 

 an hygrometer should from its first adjustment, be re-adjusted at the end of 3 

 months, and again at the end of 6 months from the first; after that, at the in- 

 terval of about 6 months, to the end of 2 years from the beginning ; and after 

 that he apprehends that once a year will suffice. The best time of adjustment, 

 being in the dry and warm weather of July or August : and by these means, he 

 apprehends the instrument will be always kept within 2° of its proper point. ' 



Mr. S. is aware that an hygrometer actuated by any principle of the kind here 

 made use of, may not be a measurer of the quantity of moisture, actually dis- 

 solved in, and Intimately mixed with the air ; but only indicates the disposition 

 of the air to part with, or precipitate the water contained in its substance; or, on 

 the contrary, to dissolve and imbibe a greater quantity : but as it is by separating 

 the effects of natural causes, that we are enabled to judge of these causes, and 

 thence their effects when again compounded; every attempt to ascertain the 

 operations of a simple cause will have its value in the search into nature : nor can 

 we a priori determine the value of any new instrument; for if it should lead to 

 a single discovery, or even to ascertain a single fact, this may again lead to 

 others of great importance, of which we might have, either none, or an imper- 

 fect idea of before. For his own part, he has always looked on a thick fog, and 

 the sweating, or condensation of the water's vapours on the walls in the inside of 

 buildings, to be the greatest marks of a moist atmosphere ; whereas he has not 

 always found the hygrometer affected at these times in the highest degree. On 

 the contrary, at the close of a fine day, and the fall of the dew on the sudden 

 approach of a frost, he has found the hygrometer more affected by moisture, 

 than in some of the preceding cases ; and still more by a falling dew in the time 

 of a hard frost. 



XXV. Letter from Mr. John Baptist Beccaria, of Turin, F. R. S., to Mr. J. 

 Canton, F. R. S., on his New Phosphorus receiving Several Colours, and only 

 Emitting the Same. From the Latin, p. 212. 



Mr. B. made several cylindrical boxes of thin iron plates, black withinside ; 

 and covered with a lid; in which was inserted a glass of any colour different from 



