114 DR MILLER ON THE METEOROLOGY OF 



periods, it sometimes, though very rarely, reaches lo^hs of an mcn m 24 hours. 

 On the 22d of May, 1844, 0-430 inch was measured, and on a freezing mixture 

 being applied to Daniell's hygrometer, the dew-point was found at 24 below the 

 temperature of the air, which was 63".* The evaporation for the month was 

 6-280 inches, with only a quarter of an inch of rain. 



Surprise is often expressed that vegetation is enabled to retain its vitality, 

 and even to come forth unscathed from the scorching ordeal to which it is occa- 

 sionally subjected, in the continuous absence of rain for weeks, and even for 

 months, as was the case in the spring of 1852, when the entire atmospheric pre- 

 cipitation in 70 days only amounted to jgths of an inch of water. But Nature has 

 provided a check, which prevents the extreme heating and aridity from which the 

 ground would otherwise suffer under such circumstances. In summer, the earth's 

 surface is not unfrequently heated by the sun's rays to 100" or upwards, even in this 

 latitude ; but as the soil is a very bad conductor of heat, this temperature does 

 not penetrate more than a very few inches downwards ; and, at a moderate depth, 

 the day and night temperatures are nearly identical. When the soil has once 

 become thoroughly desiccated, it loses that capillary action by which excess of 

 water is ordinarily withdrawn from it ; in other words, evaporation ceases, and 

 the subjacent moisture is thenceforth stored up for the special uses of the vege- 

 table kingdom : it is absorbed by the roots of trees, shrubs, plants, and grasses, 

 contributing to their growth and sustentation, as it slowly passes through their 

 vascular structure into the atmosphere. When rain at length visits the thirsty soil, 

 it does not recover its absorptive powers all at once ; in the meantime, slight 

 showers are forthwith transformed into vapour as they descend on the heated 

 ground, while heavy rains flow off from the baked and indurated surface into the 

 adjacent hollows, drains, and water-courses. 



Evaporation is scarcely ever entirely suspended, either during the heaviest 

 rains, or when the air is apparently saturated with vapour ; at least, I have met 

 with very few instances in which some daily loss was not appreciable to a finely 

 graduated instrument. This important natural process is also active at very low 

 temperatures of the air ; and it goes on freely from the surface of frozen water, 

 even when the whole mass is converted into a solid block of ice. From the 11th 

 to the 16th of December, 1846, during which a brisk breeze prevailed, the loss from 

 the frozen contents of my evaporation gauge was 0*450 inch, or -075 per diem, the 

 average for the month being -033 per diem. In twelve days of frost in February, 

 1847, the evaporation from the ice was 0-552 inch, or -046 per day, the average daily 

 quantity for the month being -030 inch. During ten days of keen frost between 



* Whilst I am revising this paper, (April 21, 1854) Evaporation is unusually active for the sea- 

 son. The loss from the gauge in the 24 hours preceding 9 a.m., was 0*30 inch, which is the great- 

 est daily quantity I have recorded in the month of April. During the last 3 days, the maximum 

 temperature has varied from 65° to 73°, and the complement of the dew-point has ranged from 

 23°*3 to 25°-5 — approaching to the extreme of hygroscopic dryness in this climate. 



