August 19, 1887.] 



SCIENCE. 



95 



wasp's flight started from a crotch in a Hmb, it is possible that the 

 locust was left in the crotch. The whole incident showed a per- 

 fect understanding, on the part of the wasp, of what he proposed 

 to do, and the carr)-ing-out of a preconceived plan of procedure 

 without any stopping to think what he would do next. The only 

 pauses were in going up the trunk of the tree. 



C. G. ROCKWOOD, Jr. 



New York, Aug. ii. 



A Good Word. 



I SEE by your last issue that the Teachers' National Association 

 indorsed the Blair bill. I am sony to learn of this, as I think that 

 bill is an imposition upon the intelligence of the people of this 

 countr)'. 



In the first place, any State that cannot support schools in which 

 to educate its children must be poor indeed ; and, in the second 

 place, any State that would accept national aid has not the spirit 

 necessary to a sound government. We can plainly see where the 

 most of that aid would go, and we do not feel like sending it into 

 those States. I am aware that many will deem me unjust ; but, 

 be that as it may, I would never consent to the Blair bill, and I am 

 sorry that my fellow-teachers ever gave their indorsement to such a 

 bill, as by so doing it may have some weight in the future ; but 

 then teachers are only mortals, and many of them seem to have 

 very poor judgment. 



I am glad to see the position that Science takes in this matter, 

 and you may rest assured that I shall be a life subscriber to that 

 paper. I consider it the best paper published in this country for 

 any advanced teacher or scientific man. I wish you the best of 

 success. James Lawrey. 



Fremont City, Ic, Aug. 8. 



The Formation and Dissipation of Sea-Water Ice. 



Sea-water possesses several characteristics that make the 

 operation of freezing different from that in the case of fresh 

 water. 



The density of sea-water increasing till the freezing-point is 

 reached, it follows that its conversion into ice will take place 

 beneath, instead of at the surface as in the case of fresh water. 

 The freezing-point in most cases, then, should be situated near the 

 bottom of the column of water, if not actually at it. 



In equal thicknesses of fresh and sea water ice, two inches of the 

 first will support a greater weight, without fracture, than an equal 

 thickness of the latter ; although it is quite possible, that, where 

 greater thicknesses are concerned, the advantage may be in the 

 opposite direction. 



Sea-water ice is much less ' brittle ' than that of fresh water ; 

 rising and falling under the influence of a heavy sea, and adapting 

 itself to its undulations, in cases where fresh-water ice would be 

 fractured : this is particularly noticeable in the earlier stages of its 

 formation. 



An inch of newly formed sea-water ice will not support a man's 

 weight, and, in giving way beneath him, does so abruptly, without 

 any warning preliminary fissures, leaving a cleanly cut hole of the 

 same extent as the surface over which direct pressure was admin- 

 istered, and thus differs from fresh-water ice, which, on being 

 fractured in this way, carries down a large portion of the surface 

 beyond the area directly under pressure. We may therefore con- 

 clude that the cohesion amongst the particles in fresh-water ice is 

 greater than in the case of sea-water, and possibly that the arrange- 

 ment of the ice-cr}'Stals is different in each. Those in the case of 

 fresh water, forming horizontally at the surface, overlap and bind 

 each other together, whilst those from sea-water would seem to 

 arrange themselves vertically, as a comparison of the fractures in 

 each case will show. 



The formation of a film of ice over a sheet of sea-water takes 

 place indifferently, as to position, during calm weather ; but, with 

 a light breeze blowing, the permanent formation commences at the 

 windward shore ; narrow and rapidly lengthening ' streamers ' 

 form from the points of this shore ; continuing, and growing very 

 slowly narrower as it does so, it may reach a length of from four 



hundred to five hundred yards ; then parallel streamers combine, 

 till at last the entire surface is covered. A great peculiarity of the 

 proceeding is the extreme narrowness of these streamers in com- 

 parison to their great length, and the consequently great cohesion 

 that is capable of overcoming the strain that must be caused by 

 even a light wind blowing over so lengthy a surface, whilst it is 

 rising and falling to the pronounced ripple on the water's 

 surface. 



Recently formed sea-water ice is not of uniform texture through- 

 out its depth. A section of four inches would be represented bv a 

 thin layer of partially decomposed ice, looking very much like thor- 

 oughly saturated snow ; then about two inches of ' sodden ' ice,, 

 occupying a condition intermediate between that of the surface 

 film and fully formed ice, both in consistency and appearance ; and, 

 finally, the fully formed ice. having every appearance of fresh-water 

 ice. These differences in the several strata of the ice do not con- 

 tinue, once the temperature of the air becomes very much lower 

 than that of the water's freezing-point. 



When the ice is first formed in tidal waters, that portion of it 

 which is left aground above low-water mark freezes to the bottom 

 (the temperature of the air being supposed to be below the water's- 

 freezing-point) ; so that, on the water rising again, it is left there,, 

 submerged. Over this, at the surface of the water, another ice- 

 film is formed, which, on reaching the level of the submerged ice,, 

 is frozen to and remains with it in this position. This operation is 

 repeated, till the result is, that a perpendicular wall of ice forms, 

 whose outer limit is the low-water mark, terminated by a hori- 

 zontal surface shorewards at the limit of high-water mark. The 

 outcome of this peculiarity is, that the shore outline in winter 

 undergoes a complete transformation, of more or less extent in 

 accordance with the difference existing between high and low 

 water mark. In the case of a mud or sand bottom, the ice, 

 though freezing to it, possesses sufficient buoyancy to raise a film 

 of mud or sand with it each time, till it is of sufficient thickness 

 to counteract this tendency. 



The explanation of this phenomenon seems to me to be as follows : 

 in the first place, it is essential that the temperature of the air should 

 be below the freezing-point of the water ; and, in the next place, it 

 is evident that the temperature of the earth forming the bottom 

 must be above the freezing-point, else ice would form there ; still,, 

 it need not be much above it, as the water, being very nearly at the 

 temperature of its freezing-point, would reduce the surface of the 

 bottom to that point, less the increase in temperature consequent 

 on the convection of the earth's heat to that surface. The freezing- 

 point of sea-water being 26°.7, the melting-point of sea-water ice 

 zS^.S (Science, ix. No. 228, x. No. 232), then, if the temperature of 

 the bottom lies between these values, we can understand, that, 

 when the formed ice is placed in contact with it by the falling tide, 

 the temperature of the air reduces that of the water which is run- 

 ning off the ice as the tide leaves it, so that it freezes and cements 

 the ice to the bottom. To free it again requires that the tempera- 

 ture of either the air or the water should rise above 28°. 8, which, 

 with the water at 26°. 7 and the atmosphere below this point, is not 

 possible : therefore our ice remains fast to the bottom. 



Fresh water freezing, and its ice melting, at the same tempera- 

 ture, it cannot possibly freeze to the bottom ; for, granting that the 

 temperature of the water may be 32°, that of the bottom must be 

 above, both on account of the water in contact with it being at a 

 higher temperature than this, and because, even if we assume the 

 temperature at 32° throughout, that of the bottom must be above 

 this, owing to convection, as before stated. Anchor-ice does form 

 in fresh water, but not on the bottom proper, as it attaches itself to 

 bowlders or pebbles which are not themselves in perfectly continu- 

 ous connection with the bottom proper, and are therefore largely 

 surrounded by the water, and correspondingly affected by its 

 temperature, whilst insensibly affected by convection ; so that, if 

 we can assume conditions under which the water's temperature 

 would be below the freezing-point, we have those cases in which 

 anchor-ice will form. 



On account of the position of the freezing-point in a column of 

 sea-water, it is possible, under certain conditions, for two films of 

 ice to form, one below the other. This was actually observed to 

 have taken place under the following conditions : the temperature 



