224 



NA TURE 



\yan. 3, 1889 



crystallography, which must at present, unfortunately, be regarded 

 as a branch of mineralogy. And if they cannot acquire sound 

 knowledge, how can ihey teach well ? I feel quite sure that if 

 the Science and Art Department would institute a summer course 

 for teachers, where they would not have to waste their time over 

 the merest rudiments, but could study practically the methods of 

 crystal measurements, and the higher branches of mineralogical 

 research, it would be largely taUen advantage of by teachers 

 and those who intend to become teachers. Failing this, I for 

 one should be glad to know of any institution in this country or 

 in Germany where such instruction could be obtained during the 

 summer months. A Teacher. 



Glasgow, December 31, 1888. 



Ventilating Bees. 



I DO not know whether it is generally known that here, and 

 I believe in other tropical countries, there are in every hive what 

 one can only describe as " ventilating bees." I mean that during 

 the hot season two or three bees post themselves, on their heads, 

 at the entrance of the hive, and fan the interior with the in- 

 cessant motion of their wings. They are relieved at intervals 

 by fresh bees, who carry on the process. They are kept to their 

 duty by a sort of patrol of bees to insure their incessant activity. 

 This is a well-authenticated and known fact, and as such may 

 interest your readers. EvA M. A. Bewsher. 



Mauritius, November 21. 



Sonorous Sand at Botany Bay. 



Referring to the correspondence with regard to the so-called 

 "musical sands," which has appeared in your columns, it may 

 be of interest to record the fact that sand with similar properties 

 is found in Botany Bay, New South Wales, not far from the spot 

 where Captain Cook first landed. When displaced by pressure 

 from above, or disturbed with the hand or stick, this sand emits 

 a musical sound, which appears to vary in intensity according to 

 the degree of moisture which it contains. Should any of your 

 correspondents wish for specimens, I shall be happy to forward 

 a small quantity to them. A. Sid.ney Ollifk. 



Australian Museum, Sydney, November 16, 1888. 



HO W RAIN IS FORMED} 



T N certain villages in the Indian Central Provinces, besides 

 ■*- the village blacksmith, the village accountant, the vil- 

 lage watchman, and the like, there is an official termed 

 the. gdpogdri, whose duty it is to make rain. So long as 

 the seasons are good and the rain comes in due season, 

 his office is no doubt a pleasant and lucrative one. It is 

 not very laborious, and it is obviously the interest of all 

 to keep him in good humour. But if, as sometimes 

 happens, the hot dry weather of April and May is pro- 

 longed through June and July, and week after week the 

 ryot sees his young sprouting crops withering beneath the 

 pitiless hot winds, public feeling is wont to be roused 

 against the peccant rain-maker, and he is led forth and 

 periodically beaten until he mends his ways and brings 

 down the much-needed showers. 



You will hardly expect me, and I certainly cannot 

 pretend, to impart to you the trade-secrets of the pro- 

 fessional rain maker. Like some other branches of 

 occult knowledge which Madam Blavatsky assures us 

 are indigenous to India, this art of rain-making is perhaps 

 not to be acquired by those who have been trained in 

 European ideas ; but we can at least watch and interrogate 

 Nature, and learn something of her method of achieving 

 the same end ; and if her scale of operations is too large 

 for our successful imitation, we shall find that not only is 

 there much in it that may well challenge our interest, but 

 it may enable us to some extent to exercise prevision of 

 its results. 



Stated in the most general terms, Nature's process of 

 rain-making is extremely simple. We have its analogue 



' A Lecture delivered by H. F. Blanford, F.R.S., at the Hythe School of 

 Mur,ketry on November ij. 



in the working of the common still. First, we have steam 

 or water vapour produced by heating and evaporating the 

 water in the boiler ; then the transfer of this vapour to a 

 cooler ; and finally we have it condensed by cooling, and 

 reconverted into water. Heat is communicated to the 

 water to convert it into vapour, and when that heat is 

 withdrawn from it, the vapour returns to its original liquid 

 state. Nature performs exactly the same process. 



In the still, the water is heated until it boils ; but 

 this is not essential, for evaporation may take place 

 at all temperatures, even from ice. A common little 

 piece of apparatus, often to be seen in the window of the 

 philosophical instrument maker, and known as Wollaston's 

 cryophorus, is a still that works without any fire. It con- 

 sists of a large glass tube with a bulb at each end, one 

 of which is partly filled with water : and, all the air having 

 been driven out of the tube by boiling the water, it is 

 hermetically sealed and allowed to cool. It then contains 

 nothing but water and water vapour, the greater part of 

 which re-condenses when it cools. Now, when thus cold, 

 if the empty bulb be surrounded by ice, or, better, a 

 mixture of ice and salt, the water slowly distils over, and 

 is condensed in the colder bulb, and this without any 

 heat being applied to that which originally contained the 

 water. And this shows us that all that is necessary to 

 distillation is that the condenser be kept cooler than the 

 evaporater. 



Nevertheless, at whatever temperature it evaporates, 

 water requires heat, and a large quantity of heat, merely 

 to convert it into vapour ; and this is the case with the 

 cryophorus ; for if the evaporating bulb be wrapped round \ 

 with flannel, and so protected from sources of heat around, J I 

 the water cools down until it freezes. That is to say, it- ; 

 gives up its own heat to form vapour. A simple experi- 

 ment that anyone may try with a common thermometer 

 affords another illustration of the same fact. If a 

 thermometer bulb be covered with a piece of muslin, and 

 dipped into water that has been standing long enough to 

 have the same temperature as the air, it gives the same 

 reading in the water as in the air. But if when thus 

 wetted it be lifted out and exposed to the air, it begins to 

 sink at once, owing to the evaporation of the water from 

 the wet surface, and it sinks the lower the faster it dries. 

 In India, when a hot wind is blowing, the wet bulb 

 sometimes sinks 40° below the temperature of the air. 



Now this is a very important fact in connection with 

 the formation of rain, because it is owing to the fact that 

 water vapour has absorbed a large quantity of heat— which 

 is not sensible as heat, but must be taken away from it 

 before it can be condensed and return to the liquid state — 

 that vapour can be transported as such by the winds for 

 thousands of miles, to be condensed as rain at some 

 distant part of the earth's surface. 



I have said that the quantity of absorbed heat is very 

 large. It varies with the temperature of the water that is 

 evaporating, and is the greater the lower that temperature. 

 From water that is on the point of freezing it is such that 

 one grain of water absorbs in evaporating as much heat 

 as would raise nearly 5^ grains from the freezing to the 

 boiling point. This is called the latent heat of water 

 vapour. As I have said, it is quite insensible. The 

 vapour is no warmer than the water that produced it, and 

 this enormous quantity of heat has been employed simply 

 in pulling the molecules of water asunder and setting them 

 free in the form of vapour, which is merely water in the 

 state of gas. All liquids absorb latent heat when they 

 evaporate, but no other known liquid requires so much as 

 water. 



Many things familiar in everyone's experience find their 

 explanation in this absorption of latent heat. For instance, 

 we feel colder with a wet skin than with a dry one, and wet 

 clothes are a fruitful source of chills when the body is in 

 repose ; although, so long as it is in active exercise and 

 producing a large amount of heat, since the evaporation 



