LOCAL VARIATIONS. 51 



thai that of art may be much more considerable than 

 those who have not studied the subject can be aware of. 

 In our own country there are many instances of im- 

 provement and also of deterioration arising from small 

 operations; and there is no question that were the 

 operations extended, the result would extend in a 

 much greater proportion. The practical use should 

 be the grand object in view in the whole investigation 

 of nature; and the man who can point out how a 

 season or a district is to be ameliorated, performs a 

 far more important service than he who points out 

 how any number of plants or animals may be screened 

 from the one, or how the kinds of the one or the other 

 may be made better. That, however, must be the con- 

 tinued labour of many inquirers ; and all that we can 

 do, is entering a caveat in favour of the local and tem- 

 porary causes, (some of which we shall notice after- 

 wards,) to cast a rapid glance on the whole. 



The momentary action of the sun may, then, be 

 represented by a dome extending over that half of the 

 earth's surface, of which the point where the sun is 

 right over head is the centre, elevated there to a certain 

 height, (say 15,000 or 16,000 feet,) and descending by 

 a regular curve till it touch the surface all round the 

 circumference of the hemisphere. o far as the mo- 

 mentary action is concerned, the whole of the atmos- 

 phere below this imaginary dome will be above the 

 temperature of 32, or that at which water becomes 

 ice ; and the space above the dome, at or below 32. 

 The measures, from the upper surface of the dome to 

 the several points upon the earth's surface, estimated 

 upon lines drawn to the earth's centre, will represent 



