360 SCIENCE OF AGRICULTURE. Part II. 



23^4- In order to comjxire tlm qnantitks of rain colkcted in pluvloineters at different 

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

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

 place. 



2365. 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 per- 

 ceived in the barometer. 



236o. The scales of different thermometers axe as follow. In Fahrenheit's the freezing point is 32 degrees, 

 and the boiling point 212 degrees. In Reaumur's tlie freezing point is 0, and the boiling point 80 degrees. 

 In the centrigrade 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 contains 2j de- 

 grees of Pahrenheit, and to convert the degrees of the one to the other, the rule is to multiply by 9, divide 

 by 34, and add 32. One degree of the centigrade scale is equal to one degree and eight-tenths of Fahren- 

 heit ; and the rule 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 boihng water for the boiling point, and 

 if the space between these points is equally divided, the thermometer is correct. 



2367. The study of tlie weather froTn precedent, anords 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 

 drv, at least five times in six. 



That when a storm happens from an easterly point, either on the 19th, 20th, or Slst of May, the suc- 

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



That when a storm arises on the 25th, 26th, or 2^th 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 not, 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 Mav than September; the chances that it does so, are, at least, 4 to 3 ; but, when it rains 

 plentifully in May, as 18 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. 



2368. The probabilities ofparticvJar seasons beingfollowed by others, has 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 six wet springs, 22 dry, and 13 variable j 20 wet summers, 16 dry and 

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



A season is accounted wet, when it contains two wet months. In general, the quantity of rain, which 

 falls in dry seasons, is less than five inches, in wet seasons more ; variable seasons are those, in which 

 there falls between 30 lbs. and 36 lbs,, a pound being equal to "157639 of an inch. 



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 ia necessary for the production of 

 cora 



Sect. III. Of the Climate of Britain, 



2369. Tlie 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 coasts 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 of the water. On the 

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

 part of their heat in passing over a surface so much lower in temperature. The vari- 

 able nature of our climate is chiefly owing to the unequal breadths of watery surface 

 which surround us ; on one side, a channel of a few leagues in breadth ; on the other, 

 the Atlantic ocean. 



2370. The British climate varies materially within itself: some districts are dry, as the 

 east ; others moist, as the west coast ; in the northern extremity, dry, cold, and windy ; 

 in the south, warm and moist. Even in moist districts some spots are excessively dry, 

 as part of Wigtonshire, from the influence of the Isle of Man, in warding off the 

 watery clouds of the Atlantic ; and, in dry districts, some spots are moist, from the 

 influence of high mountains in attracting and condensing clouds charged with watery 

 vapor. 



