RAIN. 



make tlie hygrometer move towards humidity, in pro- 

 poition as the vapour is more or lefs abundant in the air : 

 on a diminution of heat, the humidity, as ftieu'n by tlie 

 hygrometer, mull intreafe ; and on an increafe of the heat, 

 the humidity mull diminifli ; and the introdudtion of other 

 hygrofcopic fubllances, drier than the air, mud have the 

 fame efFedl as an augmentation of heat. Thcfe are the pro- 

 perties of vapour, on every hypothefis of evaporation ; and, 

 therefore, all the water that exifts in the atmofphere, with- 

 out poflefling thcfe properties, is no longer vapour, but 

 mult have changed its nature. M. de Luc (hews, that 

 the water which forms rain, though it has ever been 

 confidercd and reafoned upon as producing humidity, does 

 not pofTefs tliis property, and mull, therefore, have palled 

 into another Hate. As he thinks tliat the vapour padcs 

 into an invifible ftate in the interval between evaporation 

 and its falling again in rain, and that in this Hate it is not 

 fenfible to the hygrometer, he confiders the laws of hydro- 

 logy as infulEcient for explaining the formation of rain ; 

 but he does not pretend to have difcovered the immediate 

 caufe of tlie formation of clouds and rain. If it is not in 

 the immediate produft of evaporation that rain has its 

 fource ; if the vapours change their nature in the atmo- 

 fphere, fo as no longer to be fenfible to the hygrometer, or 

 to the eye ; if they do not become vapour again till clouds 

 appear ; and if, when the clouds are formed, no alteration 

 is perceived in the quality of the air ; we muft acknowledge 

 it to be very probable, that the intermediate ftate of vapour 

 is no other than air, and that the clouds do not proceed 

 from any diftinft fluid contained in the atmofphere, but 

 from a decompofition of a part of the air itfelf, perfeAly 

 iimilar to the reft. For other obfervations on the afcent 

 and nature of vapour, and the fubfcqucnt formation of 

 rain, we refer to the articles Meteorology, and Va- 



POUH. 



As to the general quantity of rain that falls, and its pro- 

 portion in feveral places at the fame time, and in the fame 

 place at feveral times, we have many obfervations, journals, 

 &c. in the Memoirs of the French Academy, the Philof. 

 Tranf. and many other publications ; from which we (hall 

 make the followmg extrafts. 



Upon meafuring the rain falling yearly, its depth, at 

 a medium, is found as m the following table. 



the whole quantity of ram at London, in each of the 

 years fpecificd below, was as follows : viz. 



Inches. 

 26.328 



24-083 



20.354 



'774 

 '775 

 1776 

 1777 

 1778 

 1779 

 1780 



25-371 



20.772 

 26.785 



'7-3'3 



Proportid 



Medium of thcfe fevcn years 23.001 



1 of the Rain of the feveral Seafons 

 another. 



to one 



See Philof. Tranf. abr. vol. iv. pt. ii. p. 81, &c. and alfo 

 Meteorological Journal of the Royal Society, publifhed 

 annually in the Phil. Tranf. 



As rain-gages have been fixed of late years in almoft; 

 every part of the kingdom, we are enabled to determine, 

 with confiderable exadlnefs, the depth of water whicli the 

 rain yields in any given place. It may be obferved, luiwever, 

 that inland counties have lefs rain than maritime ones, efpe- 

 Deptli of Rain falling yearly, and its Proportion in feveral cially thofe which border on the weltern feas. But ftill a 



Places. 



Inches. 

 At Townley, in Lancafliire, obferved by Mr. Townley 42^ 

 Upminfter, in Ellex, by Dr. Derham ... - ig^ 

 Zurich, in Swilferland, by Dr. Scheuchzer - - - 32^; 

 Pifa, in Italy, by Dr. Mich. Ang. TiUi - - - 43^ 

 Paris, in France, by M. de la Hire - . - . . ig 

 Lifle, in Flanders, by M. de Vauban 24 



greater difference feems to take place between a mountainous 

 country, and a champaign or flat country. In the former 

 there often falls double or triple the quantity of rain in a 

 year that there does in the latter, and never lefs than an 

 equal quantity. It is alfo obferved, that thofe winds bring 

 moft rain that blow from the quarter in which is the molt 

 and neareft fea ; as are well and louth-weft winds. The 

 rain-gage alfo fliews, that more rain is collefted in the 

 inftrument, as it is placed nearer the ground ; without any 

 appearance of a difl^erence, between two places, on account 

 of their difference of level above the fea, provided that the 

 inftrument is as far from the ground at the one place, as it 

 is at the other. Thefe efletls are noticed in the Phil. Tranf. 

 for 1769 and 1771, the former by Dr. Heberden, and the 

 latter by Mr. Daines Barrington. Dr. Heberdeen fays, 

 " A compai-ifon having been made between the quantity of 

 rain which fell in two places in London, about a mile 

 diilant from one another, it was found that the rain in one 

 of them conftantly exceeded that in the other, not only 

 every month, but almoft every time that it rained. The ap- 

 paratus ufed in each of them was very exadl, and both made 

 From the Meteorological Journal of the Royal Society, by the fame artift ; and upon examining every probable 

 kept by order of the prefident and council, it appears that caufe, this unexpedled variation did not appear to be owing 



to 



Mediums 19.14 



20.19 



