ya;/. 4, 1872 



NATURE 



189 



visionary foundation than that which rests on peripolar 

 molecules seeing that it rests upon structural facts which 

 cannot be called in c|uestion a theory also which, as 

 will be seen in due time, has this in its fa\ our. — that it 

 will simplify not a little several important problems in 

 ])hysioloyy. 



C. li. Radclifke 



ICE-MAKING IN THE TROPICS 



'T^HK most marked example of the influence of radia- 

 -•- tion of heat on temperature is its influence on the 

 production of artificial ice by the natives of India. 



The fields in which the ice is made are low, flat, and 

 open ; and the ice is produced in large quantities when 

 the temperature of the air is 16 or 20° F. above the freezing 

 point ; and the plan followed is an interesting example of 

 accurate observation applied to practical purposes by a 

 people now ignorant of science. The same process lias 

 been employed from time immemorial in India with, 

 scientific accuracy ; and while the theory was explained by 

 Dr. WelU," the practical application was not so well 

 understood ; and this first led me to investigate the sub- 

 ject in India. f 



The following method is employed by the natives of 

 Bengal for making ice at the town of Hooghly near 

 Calcutta, in fields freely exposed to the sky, and formed 

 of a black loam soil upon a substratum of sand. 



The natives commence their preparations by marking 

 out a rectangular piece of ground 120 feet long by 20 

 broad, in an easterly and westerly direction, from which 

 the soil is removed to the depth of two feet. This exca- 

 vation is smoothed, and is allowed to remain exposed to 

 the sun to dry, when rice straw in small sheaves is laid 

 in an oblique direction in the hollow, with loose straw upon 

 the top, to the depth of a foot and a half, leaving its surfjce 

 half a foot below that of the ground. Numerous beds of 

 this kind are formed, with narrow pathways between them, 

 in which lar.;e earthen water-jars are sunk in the ground 

 for the convenience of having water near, to fill the shallow 

 unglazed earthen vessels in which it is to be frozen. Thebe 

 dishes are 9 inches in diameter at the top, diminishing 

 to 4f'jj inches at the bottom, \ l\^ deep, and /j, of an inch 

 in thickness ; and are so porous as to become moist 

 throughout when water is put into them. 



During the day the loose straw in the beds above the 

 sheaves is occasionally turned up, so that the whole may 

 be kept dry, and the water-jars between the beds are 

 filled with soft pure water from the neighbouring pools. 

 Towards evening the shallow earthen dishes are ar- 

 ranged in rows upon the straw, and by means of small 

 earthen pots, tied to the extremities of long bamboo rods, 

 each is filled about a third with water. The quantity, 

 however, varies according to the expectation of ice — which 

 is known by the clearness of the sky, and the steadiness 

 with which the wind blows from the N. N.W. When favour- 

 able, about eight ounces of water is put into each dish, and 

 when less is expected, from two to four ounces is the 

 usual cjuantity ; but, in all cases, more water is put into 

 the dishes nearest the western end of the beds, as the sun 

 first falls on that part, and the ice is thus more easily re- 

 moved, from its solution being quicker. 



There are about 4,590 plates in each of the beds last 

 made, and if we allow five ounces for each dish, which 

 presents a surface of about 4 inches square, there will 

 be an aggregate of 239 gallons, and a surface of 1,530 

 square feet of water in each bed. 



In the cold season, when the temperature of the air at 

 the ice-fields is under 50' F., and there are gentle airs 

 from the northern and western direction, ice forms in the 

 course of the night in each of the shallow dishes. Persons 



* Essay f 

 t Experir 



Jovir. As Society, Caliult.i 



are stationed to observe when a small film appears upon 

 the water in the dishes, when the contents of several are 

 mixed together, and thrown over the other dishes. This 

 operation increases the congealing process ; as a state of 

 calmness has been discovered by the natives to diminish 

 the quantity of ice produced. When the sky is quite clear, 

 with gentle steady airs from the N.N.W., which proceed 

 from the hills of considerable elevation near Bheerboom, 

 about 100 miles from Hooghly, the freezing commences 

 before or about m'dnight, and continues to advance until 

 morning, when the thickest ice is formed. I have seen it 

 seven-tenths of an inch in thickness, and in a few very 

 favourable nights the whole of the water is frozen, when 

 it is called by the natives solid ice. When it commences to 

 congeal between two and three o'clock in the morning, 

 thinner ice is expected, called paper-ice ; and when about 

 four or five o'clock in the morning the thinnest is obtained, 

 called flower-ice. 



Upwards of two hundred and fifty persons, of all ages, 

 are actively employed in securing the ice for some hours 

 every morning that ice is procured, and this forms one of 

 the most animated scenes to be witnessed in Bengal. In a 

 favourable night upwards of 10 cwt. of ice will be obtained 

 from one bed, and from twenty beds upwards of 10 tons. 



When the wind attains a southerly or easterly direction, 

 no ice is formed, from its not being sufficiently dry ; not 

 even though the temperature of the air be lower than 

 when it is made with the wind more from a northern or 

 western point. The N.N.W. is the most favourable di- 

 rection of wind for making ice, and this diminishes in 

 power as it approaches the due north, or west. In the 

 latter case more latitude is allowed than from the N.N.W. 

 to the north. .So great is the influence of the direction of 

 wind on the ice, that when it changes in the course of a 

 night from the N.N.W. to a less favourable direction, the 

 change not only prevents the formation of more ice, but 

 dissolves what may have been formed. On such oc- 

 casions a mist is seen hovering over the ice-beds, from 

 the moisture over them, and the quantity condensed by 

 the cold wind. A mist in like manner forms over deep 

 tanks during favourable nights for making ice. 



Another important circumstance in the production of 

 ice is the amount of wind. When it approaches a breeze 

 no ice is formed. This is explained by such rapid cur- 

 rents of air removing the cold air, before any accumulation 

 of ice has taken place in the ice beds. It is for these 

 reasons that the thickest ice is expected when during the 

 day a breeze has blown from the N.W., which thoroughly 

 dries the ground. 



The ice-dishes present a large moist external surface to 

 the dry northerly evening air, which cools the water in 

 them, so that, when at 61', it will in a few minutes fall to 

 56°, or even lower. But the moisture which exudes 

 through the dish is quickly frozen, when the evaporation 

 from the external surface no longer continues radiative ; a 

 more powerful agent then produces the ice in the dishes. 



The quantity of dry strav/ in the ice-beds forms a large 

 mass of a bad conductor of heat, which penetrates but a 

 short way into it during the day ; and as soon as the sun 

 descends below the horizon, this large and powerfully- 

 radiating surface is brought into action, and atTects the 

 water in the thin porous vessels, tnemselves powerful 

 radiators. The cold thus produced is further increased by 

 the damp night air descending to the earth's surface, and by 

 the removal of the heating cause, which deposits a portion 

 of its moisture upon the now powerfully radiating, and 

 therefore cold surface of the straw, the water, and the 

 large moist surface of the dishes. When better radiators of 

 heat were substituted, as glazed, white, or metallic dishes, 

 the cold was greater, and the ice was thicker, and the 

 dishes were heavier in the morning than the common 

 dishes. Any accumulation of heat on their surface 

 from the deposit of moisture is prevented by the 

 cold dr>- north-west airs which slowly pass over the 



