It is believed that during the passage of a hail storm, the temperature of the upper air is considerably below the mean. The simple presence of clouds in the lower atmosphere would tend to produce such an effect. The atmosphere derives its heat from the earth, and is but little affected by the direct passage of the solar rays. The heat which the earth imbibes from the sun is continually thrown off by radiation;-but when the surface of the earth is covered by a cloud, this radiant heat is intercepted, and the temperature of the lower air is thereby elevated. On a still night, the presence of clouds sometimes causes the thermometer to stand ten or fifteen degrees higher that it would otherwise. But if, by the interposition of a cloud, the lower atmosphere becomes unusually hot, the atmosphere above the cloud must receive less than its usual supply of heat, that is, it must become unusually cold.

Moreover, in the storm of July 1st, the hail was formed in a current blowing violently from the northwest, which came therefore from a higher latitude, and of course brought with it a diminished temperature. I have no data sufficiently precise for estimating the effect to be ascribed to this cause, but I think we may conclude that at the time of the storm in question, at an elevation of 5000 feet above New York, the temperature could not have differed much from 32°. We have not however yet reached the temperature necessary to the production of hail.

Another source of cold is to be found in the evaporation from the surface of the hailstones. It is well known that if we tie a piece of thin muslin upon the bulb of a thermometer, and then after dipping the bulb in water, swing it rapidly through the air, the thermometer will sink below the temperature of the air, several degrees, sometimes ten or fifteen; an effect due to evaporation. During a hail storm, the hot air from the earth's surface is carried by the upward movement to a considerable elevation, by expansion it is cooled, and a portion of its vapor is condensed. The drops thus formed at a temperature not far from 329, are still further cooled by evaporation from their surface, (the evaporation being promoted by their rapid motion;) the remainder of the drop is congealed; and as new vapor is precipitated, it is congealed upon the former;-thus forming concentric layers round the nucleus. Since water, like nearly every other substance, in passing to the solid state, inclines to crystallization, the ball as it increases, does not generally retain the spherical form, but shoots out irregular prisms. How does a hailstone remain suspended in the air long enough to acquire a weight of half a pound?

The

This difficulty is not, to my mind, a very formidable one. I conceive that hail stones are formed with great rapidity. vapor is condensed with great suddenness and almost instantly frozen. I think very large hailstones may be formed in five

minutes. In a vacuum, a stone would fall from the height of 5000 feet in less than 20 seconds-but drops of water and hail stones fall with only a moderate velocity. From my own observations of the hail stones of July 1st, I estimated the velocity of their fall at about 40 feet per second. At the uniform rate of 40 feet per second, a stone would be more than two minutes in falling 5000 feet.

In order to obtain some reliable data for estimating the velocity of hail stones, I have computed the greatest velocity of a number of small bodies differing in size and specific gravity. Dr. Hutton determined by numerous experiments the resistance of the air to bodies moving with different velocities; and in the third volume of his Tracts, p. 218, has given a table of the air's resistance to a sphere 2 inches in diameter. His experiments also indicated that the resistance, of spheres increases in a ratio somewhat greater than the squares of the diameters. This excess for spheres of from 2 to 4 inches diameter was about th part of the resistance. The second column in the following Table is taken from Hutton's Tracts, the resistances for velocities from 50 to 100 being supplied by interpolation. The resistance for a sphere 1 inch in diameter is found by taking one-fourth of the numbers in the second column, and diminishing the result by one-thirtieth part. Each succeeding column is derived from the preceding in a similar manner.

Resistance of the air to Spheres of different Diameters.

Velocity Sphere 2 inch- Sphere 1 inch Sphere inch Sphere inch Sphere inch per second es in diameter. in diameter. in diameter. in diameter.

in diameter.

In a vacuum, a body falling under the influence of gravity is continually accelerated; but when a heavy body falls through the atmosphere, the resistance increases with the velocity, until the resistance becomes equal to the weight of the body. When this takes place, there can be no further increase of velocity, and the body will afterwards descend with a uniform motion. In order therefore to determine the greatest velocity which a heavy body can acquire by falling through the atmosphere, it is only necessary to compute the weight of a sphere of given diameter, and then to search in the preceding table for the velocity due to an equal resistance upon a body of the proposed diameter. The following Table exhibits the results for spheres of lead (assuming the specific gravity 11:35), of iron (specific gravity 7·78), of water, of ice (sp. gr. 0.93), and cork (sp. gr. 025); the diameters. varying from two inches to one-eighth of an inch.

Thus it appears that a hail stone in the form of a sphere two inches in diameter, falling through a tranquil atmosphere cannot possibly acquire a velocity exceeding 90 feet per second, and spheres of a smaller size would acquire a still less velocity. Also a hail stone of irregular shape would experience more resistance than a sphere, and would acquire a smaller velocity in falling. An upward current of air moving with a velocity of 90 feet per second, or 61 miles per hour, would sustain a sphere of ice two inches in diameter; also an upward current of 30 miles per hour would sustain hail stones of half an inch in diameter, and would greatly reduce the velocity of stones of larger size. The strong upward movement which is known to exist in the neighborhood where hail is formed, is therefore quite sufficient to sustain hail stones of the largest kind as long as they can be kept within the influence of this vortex. After they have entirely escaped from the influence of the vortex, small stones would fall to the earth from an elevation of 5000 feet in about two minutes; and very large stones in one minute. I see no difficulty therefore in supposing the great mass of hail to remain in the air five minutes before reaching the earth-and that in peculiar cases, stones may remain supported for ten minutes and even a great deal longer. This period appears to me sufficient to account for the hail which fell at New York.

SECOND SERIES, Vol. XVII, No. 49.-Jan., 1854.

7

Why did the hail in the present case attain to such unusual size?

Because of the following circumstances which are unusually favorable to its formation. The temperature of the air before the storm was 90°, and it is my opinion that the dew-point could not have been less than 80°; in other words the atmosphere contained about as much vapor as it is ever known to contain in this latitude. This vapor was suddenly lifted to a region of great cold, and rapidly condensed and frozen. The strong upward movement helped to sustain the crystals as they increased in size, until the upward force was no longer equal to gravity-or until they escaped from the influence of the vortex. Most of the stones would fall in five minutes and be of moderate size; others might be sustained ten or fifteen minutes, and attain enormous dimensions.

How did the hail in this storm compare with the most remarkable cases on record.

There are well authenticated cases of hailstones having fallen in England and France weighing half a pound-and even more than this but the accounts of hail stones weighing so much as one pound, do not appear to me entirely satisfactory. A mass of ice of the specific gravity 0.93, weighing eight ounces must contain nearly 15 cubic inches; or would make a cube whose edge is nearly 2.5 inches. I have selected a piece of ice which was estimated to be about the size of the largest stone which I saw fall on the 1st of July, and found it to weigh eight ounces. But these large stones of July 1st appeared to me unusually white, and may therefore be conjectured to have had a spongy nucleus-which would have reduced the weight to perhaps six

ounces.

The hail therefore in the present storm was somewhat smaller than has been observed to fall in other places.

Since the preceding was written, I have received notice of several remarkable hail storms in different parts of the United States, two of which were so extraordinary that I have added an account of them to this paper.

Hail Storm experienced at Warren, N. H., Aug. 13, 1851.

My first information respecting this storm was derived from a letter from Dr. Peter L. Hoyt, dated Wentworth, Grafton Co., N. H., Aug. 3, 1853. The following is an extract from Dr. Hoyt's letter.

"Perhaps a brief notice of a hail storm which occurred in this vicinity on the 13th of August, 1851 may be of interest to you. This shower, about one o'clock, P. M., passed from the west towards the east over an extent of four or five miles around the base of Moosehillock Mountain, in the towns of Benton and Warren. The largest and most hail fell in the north east part of

the latter town, in a basin between the mountains near the source of a stream called Baker's river. I stood at the railroad depot in Wentworth, at the time of this shower, distant in an air line six or seven miles. It was the most remarkable appearing cloud I think I ever saw-so black and dense, encircling and covering the mountain, and shutting down to the earth.

"The hail was of prodigious size and in great quantities. The largest of the stones was of an irregular shape, rough and angular, suggesting the idea to some that they were made up of a number adhering together. They were however very solid and not easily broken.

"One was weighed upon the spot at the time of the shower, and weighed 20 ounces; and the person who informed me of this was of the opinion that he saw one fall and break in pieces which was still larger. They looked, he said, like vast pieces of ice that had been broken above, and were falling to the earth. A quantity was gathered in a basket and brought to Warren village, a distance of three or four miles, and there exhibited at least an hour after the shower, and in a hot and sultry afternoon. One of them there weighed 14 ounces, and measured 10 inches in circumference. Twelve of the largest taken out of the basket weighed. on the counter scales in the store, seven pounds.

"About 4 o'clock, P. M., three hours after their fall, a box of them was brought to Wentworth village, where I reside, a distance of about eight miles. One of them was shown to me. Its diameter according to my best judgment was from 2 to 2 inches. It had the appearance of being originally somewhat angular, with the angles melted off. It was perfectly solid and clear.

"So large was the quantity of hail in many places in Warren, that a cart load might have been gathered without moving from the place. Luckily the track of the storm was not through the most cultivated part of the towns, but along the borders and skirts of the forests, where the population was scattering. Crops of hay and grain were ground to the earth, poultry were killedcattle's backs were bruised-and the roofs of many buildings. were badly broken. But little glass was broken from the fact that the direction of the hail was nearly perpendicular to the earth."

Immediately upon receiving this letter, I wrote to Dr. Hoyt, stating that the facts which he had communicated respecting the size of the hail were so remarkable, that they ought to be substantiated by such evidence as would be deemed conclusive in a court of justice; that it was therefore important that he should obtain written statements from the identical persons who weighed the stones; and that it would not be derogatory to the dignity of science for these persons to make affidavit to the truth of their statements before a Justice of the Peace. I also suggested several