Book HX. OF THE ATMOSPHERE. 359 



tain this point by the following observation : A few hours before he departs, let the 

 traveller notice the mercury in the upper part of the tube of the barometer ; if 

 rain is about to fall, it will be indented, or concave ; if otherwise, convex or pro- 

 tuberant. 



. 2352. Barometer in spring. Towards the end of March, or more generally in the 

 beginning of April, the barometer sinks very low, with bad weatlier ; after which, it 

 seldom falls lower than 29 degrees 5 minutes till the latter end of September or October, 

 when the quicksilver falls again low, with stormy winds, for then the winter constitution 

 of the air takes place. From October to April, the great falls of the barometer are from 

 29 degrees 5 minutes to 28 degrees 5 minutes, and sometimes lower ; whereas during 

 the summer constitution of the air, the quicksilver seldom falls lower than 29 degrees 5 

 minutes. It therefore follows that a fall of one tenth of an inch, during the summer, 

 is as sure an indication of rain, as a fall of between two and three tenths is in the 

 winter. 



2353," Barometer relative to situation. It must, however, be observed, that these 

 heights of the barometer hold Only in places nearly on a level with the sea ; for expe- 

 riments have taught us, that for every eighty feet of nearly perpendicular height that the 

 barometer is placed above the level of the sea, thp quicksilver sinks one tenth of an inch : 

 observations alone, therefore, must determine the heights of the quicksilver, which in each 

 place denotes either fair or foul weather. 



2354. The hygrometer is of various sorts, but cord, fiddle-string, and most of the sub- 

 stances commonly used become sensibly less and less accurate, so as at length not to 

 undergo any visible alteration from the different states of the air, in regard to dryness or 

 moisture. 



2355. A sponge makes a good hi/grometer on this account, as being less liable to be 

 changed by use than cord. To prepare the sponge, first wash it in water, and when dry, 

 wash it again in water wherein sal ammoniac or salt of tartar has been dissolved ; and let 

 it dry again. Now, if the air becomes moist, the sponge will grow heavier j and if dry, 

 it will become lighter. 



2356. Oil of vitriol is found to grow sensibly lighter or heavier in proportion to the 

 lesser or greater quantity of moisture it imbibes from the air. The alteration is so great, 

 that it has been known to change its weight from three drachms to nine. The other acid 

 oils, or, as they are usually called, spirits, or oil of tartar per deliquiumy may be substi- 

 tuted for the oil of vitriol. 



2357. Steel-yard hygrometer. In order to make a hygrometer with those bodies which 

 acquire or lose weight in the air, place such a substance in a scale on the end of a 

 steel-yard, with a counterpoise which shall keep it in equilibrio in fair weather; the 

 other end of the steel-yard, rising or falling, and pointing to a graduated index, will 

 shew the changes. 



2358. Line and plummet. If a line be made of good well dried whip cord, and a 

 plummet be fixed to the end of it, and the whole be hung against a wainscot, and a line 

 be drawn under it, exactly where the plummet reaches, in very moderate weather it will 

 be found to rise above such line, and to sink below it when the weather is likely to 

 become fair. 



2359. The whalebone hygrometer^ originally invented by De Luc, is esteemed one of 

 the best now in use. 



2360. The rain-gaugey pluviometer^ or hyetometer, is a machine for measuring the 

 quantity of rain that falls. 



2361. A hollow cylinder forms one of the best-constructed rain gauges : it 249 

 has within it a cork ball attached to a wooden stern (Jig. 249. ), wliich passes 

 through a small opening at the top, on which is placed a large funnel. When 

 this instrument is placed in the open air in a free place, the rain that falls within, 

 the circumference of the funnel will run down into the tube and cause the cork 

 to float ; and the quantity of water in the tube may be seen by the height to 

 which the stem of the float is raised. The stem of the float is so graduated, as to 

 show by its divisions the number of perpendicular inches of water which fell on 

 the surface of the earth since the last observation. After every observation the 

 cylinder must be emptied. 



2362. A copper funnel forms another very simple rain-gauge : the area of the opening 

 must be exactly ten square inches. Let this funnel be fixed in a bottle, and the quantity 

 of rain caught is ascertained by multiplying the weight in ounces by 173, which gives 

 the depth in inches and parts of an inch. 



2363. Infixing these gauges, care must be taken that the rain may have free access 

 to them ; hence the tops of buildings are usually the best places, though some 

 conceive that the nearer ^he rain-gauge is placed to the ground the more rain it will 

 collect. 



Aa 4 



