Book III. OF THE ATMOSPHERE. 367 



2428. 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. 



2429. In JLvhig 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 the rain-gauge is placed to the ground the more rain it will 

 collect. 



2430. In order to compare the quantities of rain collected in pluviometers at different 

 places, the instruments should be fixed at the same heights above the ground in all such 

 places ; because, at diflEerent heights, the quantities are always different, even at the same 

 place. 



2431. Thermometer. As the weight of the atmosphere is measured by the barometer, 

 so the thermometer shows the variations in the temperature of the weather ; for every 

 change of the weather is attended with a change in the temperature of the air, which a 

 thermometer placed in the open air will point out, sometimes before any alteration is 

 perceived in the barometer. 



2432. The scales of different thermometers are as follows : In P'ahrenheit's the freezing point is 32 

 degrees, and the boiling point 2l2 degrees. In Reaumur's the freezing point is 0, and the boiling point 

 80 degrees. In the centigrade thermometer, which is generally used in France, and is the same as that 

 of Celsius, which is the thermometer of Sweden, the freezing point is 0, and the boiling point 100 degrees. 

 As a rule for comparing or reducing these scales, it may be stated, that 1 degree of Reaumur's scale con- 

 tains 2^ degrees of Fahrenheit, and to convert the degrees of the one to the other, the rule is to multiply 

 by 9, divide by 84, and add 32. One degree of the centigrade scale is equal to one degree and eight tenths 

 of Fahrenheit ; and the jule here is to multiply by 9, divide by 5, and add 32. Any of these thermometers 

 may be proved by immersing it in pounded ice for the freezing point, and in boiling water for the boiling 

 point, and if the space between thesfc points is equally divided, the thermometer is correct. 



2433. The study of the weather from precedent, affords useful hints as to the character 

 of approaching seasons. From observing the general character of seasons for a long 

 period, certain general results may be deduced. On this principle, Kirwan, on com- 

 paring a number of observations taken in England from 1677 (Trans. Ir. Acad. v. 20.) to 

 1789, a period of 112 years, found : 



That when there has been no storm before or after the vernal equinox, the ensuing summer is generally 

 dry, at least five times in six. 



That when a storm happens from an easterly point, either on the 19th, 20th, or 21st of May, the succeed, 

 ing summer is generally dry, at least four times in five. 



That when a storm arises on the '25th, 9.&th, or 21th of March, and not before, in any point, the succeed- 

 ing summer is generally dry, four times in five. 



If there be a storm at S. W. or W. S. W. on the 19th, 20th, 21st*, or 22d of March, the succeeding sum- 

 mer is generally wet, five times in six. 



In this country winters and springs, if dry, are most commonly cold ; if moist, warm : on the contrary, 

 dry summers and autumns are usually hot,"and moist summers cold ; so that, if we know the moistness 

 or dryness of a season, we can form a tolerably accurate judgment of its temperature. In this country 

 also, it generally rains less in March than in November, in the proportion at a medium of 7 to 12. It 

 generally rains less in April than October, in the proportion of 1 to 2 nearly at a medium. It generally 

 rains less in May than September; the chances that it does so are at least 4 to 3; but, when it rains 

 plentifully in May, as IS inches or more, it generally rains but little in September; and when it rains 

 one inch, or less, in May, it rains plentifully in September. 



2434. The probabilities of particidar seasons being followed by others have been calculated 

 by Kirwan ; and although his rules chiefly relate to the climate of Ireland, yet as there 

 exists but little difference between that island and Great Britain, in the general appear- 

 ance of the seasons, we shall mention some of his conclusions. 



In forty-one years there were 6 wet springs, 22 dry, and 13 variable; 20 wet summers, 16 dry, and 

 5 variable; 11 wet autumns, 11 dry, and 19 variable. 



2435. A season is accounted wet, when it contains two wet months. In general, the 

 quantity of rain, which fall in dry seasons, is less than five inches, in wet seasons more ; 

 variable seasons are those, in which there fall between 30 lbs. and 36 lbs. , a pound being 

 equal to -157639 of an inch. 



2436. January is the coldest month in every latitude ; and July is the warmest month 

 in all latitudes above 48 degrees : in lower latitudes, August is generally the warmest. 

 The difference between the hottest and coldest months increases in proportion to the 

 distance from the equator. Every habitable latitude enjoys a mean heat of 60 degrees for 

 at least two months j which heat is necessary for the production of com. 



Sect. III. Of the Climate of Britain. 



2437. The climate of the British isles, relatively to others in the same latitude, is tem- 

 perate, humid, and variable. The moderation of its temperature and its humidity are 

 owing to our being surrounded by water, which being less affected by the sun than the 

 earth, imbibes less heat in summer, and, from its fluidity, is less easily cooled in winter. 

 As the sea on our coast never freezes, its temperature must always be above 33 or 

 34 ; and hence, when air from the polar regions at a much lower temperature passes 

 over It, that air must be in some degree heated by the radiation from the water. On the 

 other hand, in summer, the warm currents of air from the south necessarily give out 



