480 PHYSICAL GEOGRAPHY. 



circumference, and another weighed two ounces. They ploughed up a gravel-walk like 

 musket balls, and passed through glass windows, making round holes, but not cracking 

 them." 



The aqueous vapour in the atmosphere assumes another form, that of dew, under 

 circumstances favourable to its production. These occur when the sun is absent, the sky 

 clear and nearly serene, and when the air, replete with moisture, is chilled by contact 

 with some surface or substance colder than itself. It was once supposed that the dew, 

 fringing the blades of grass and the leaves of the trees with silvery beads, sparkling in the 

 morning sunlight, dropped 



" Like the gentle rain from heaven." 



Hence our phrase, the drops of dew, alludes to its presumed descent from the upper strata 

 of the atmosphere. It is surprising that this should have been the popular belief down to 

 a very recent period, for dew may be seen upon an under surface which nothing falling 

 can touch, and upon a side surface, which nothing, by either rising or falling, can reach. 

 <; The dew-drop is familiar to every one from earliest infancy. Resting in luminous beads 

 upon the down of leaves, or pendant from the finest blades of grass, or threaded upon the 

 floating lines of the gossamer, its orient pearl varies in size, from the diameter of a small 

 pea to the most minute atom that can be imagined to exist. Each of these, like the rain- 

 drops, have the properties of reflecting and refracting light ; and hence, as from so many 

 minute prisms, the unfolded rays of the sun are sent up to the eye in similar brilliant 

 colours to those of the rainbow. When the sunbeams traverse horizontally a very thickly 

 bedewed grass-plot, these colours arrange themselves so as to form an iris or dew-bow ; 

 and if we select any one of the drops for observation, and steadily regard it while we 

 change our position, we shall find the prismatic colours follow each other in their regular 

 order." The poets of nature in all ages and countries have seized upon the clear, 

 tremulous, pendant, and sparkling drops of dew, as images of purity gentleness, and 

 beauty ; and on account of their service to mankind in the economy of the vegetable 

 kingdom, and the interesting mode of their formation, they claim the attention of the 

 scientific inquirer. With us, the dew is most copious in spring and autumn ; and it has been 

 produced, during a single night, in a quantity sufficient to be measured by the rain-guage. 

 This occurs chiefly in the autumn. Mr. Howard found by his instrument a deposition of 

 dew equal to one-tenth of an inch, on the morning of September 1st, 1818, the production 

 of the preceding night. Dr. Dalton estimates the annual deposit in this country to be 

 five inches, or about 22,161,237,355 tons, reckoning the ton to be equal to 252 imperial 

 gallons. 



The deposition of dew was first satisfactorily explained by Dr. Wells, in the year 1814, 

 in an Essay which has been pronounced by a high authority Dr. Thomson " one of 

 the most beautiful examples of inductive reasoning in the English language." When the 

 sun is below the horizon, and for a short period before his setting, bodies upon the sur- 

 face of the earth, exposed to the aspect of a clear sky, cool by the radiation of the particles 

 of heat absorbed, and at a more rapid rate than the atmosphere. The air in immediate 

 contact with these bodies, replete with humidity in the form of transparent aqueous 

 vapour, is chilled by their cold embrace ; and, owing to the increase of its density, it 

 becomes incapable of holding in suspension the moisture with which it is charged in the 

 same quantity as before. The surplus is therefore disengaged, and appears upon the 

 surface of the refrigerating object in globules of dew. It is essential to this process, that 

 the night should not be a cloudy one ; because when the sky is overcast, the radiant 

 caloric proceeding from the surface of the earth, and which otherwise would go off into 

 free space, is intercepted by the clouds, and returned by them in sufficient quantity to 

 prevent the decrease of temperature necessary to compel the atmosphere to surrender a 



