ON RAIN, EVAPORATION, &C J59 



A certain clafs of founds, which, for the fake of brevity, Clafs of founds 

 were not noticed in my paper on the voice, deferve a place in ^kfrcceffio" 

 the prefent communication. If a finely-toothed file pafs flowly of other founds; 

 over a fmooth elaflic fubftance, fuch as a piece of horn, it the firft g' lvin S 



•/•i t - r 1 L tne tune or * 



makes a grating none ; but if the velocity or the lnltrument bepi tc h, and the 

 fufficiently increafed, a continued found is produced, which latter tlie tone 

 becomes more or lefs acute, by giving a quicker or ilower 

 motion to the file. The grating noife is occafioned by a fuc- 

 ceffion of fhort interrupted founds, refulting from the united 

 vibrations of the file and the body it fcratches ; but the quick 

 fucceffion of thefe founds, caufed by an increafe of velocity, 

 gives rife to a fecondary found refembling the harmonical 

 notes, being produced by a like caufe. Now this found be- 

 comes a primary object with the ear, in all probability becaufe 

 the pitch of it may be varied ; for the firft founds proceeding 

 from the action of the file, evidently fupply nothing but the 

 tone. Many inftances of the kind occur in art and nature : 

 the notes of all reed-inftruments are of this defcription, and 

 the voice muft be referred to the fame clafs, becaufe the la- 

 rynx refembles a reed-inftrument in ftructure. 



VI. 



Experiments and Obfervations to determine •whether the Quantity 

 of Rain and Dew is iqual to the Quantity of Water carried of 

 by the Rivers and raifed by Evaporation ; with an Enquiry into 

 the Origin of Springs. By Mr. John Dalton*. 



T is fcarcely poffible to contemplate without admiration the Interefttng fyf. 



I 



beautiful fyftem of nature by which the furface of the earth is** 01 H w . h ^ h 



11 r i- i • i ii n • the globe is fup- 



continually iupphed with water, and that unceafing circula- p ]i e d with 



tion of a fluid fo effentially neceflary to the very being of the watcr » 

 animal and vegetable kingdoms lakes place. Naturalifts, 

 however, are not unanimous in their opinions whether the rain 

 that falls is fufficient to fupply the demands of fprings and ri- 

 vers, and to afford the earth befides fuch a large portion for 

 evaporation as it is well known is raifed daily. To afcertain whether the 

 this point is an object of importance to the fcience of agricul- rain be equal to 

 ture, and to every concern in which the procuration and Jj rivers™^.' 



# Manchefter Memoirs, V. 346*. 



management 



