December 19, 1890.] 



SCIENCE. 



345 



published. This notice itself requires a notice here, inasmuch as 

 it aims a blow, not only at all recent advancement in cyclonology, 

 but even at its very foundation in Espy's condensation theory. 

 This notice has been long delayed from a desire to first see. and to 

 reply to, Dr. Hann's memoir, which could not be found either in 

 Washington or Boston. In this memoir, as the reader has been 

 advised. Dr. Hann takes a new position with regard to the origin 

 of cyclones; namely, that they depend upon the same forces, aris- 

 ing from the difference of temperature between the equatorial and 

 polar regions, upon which the general circulation does, and that they 

 are therefore simply subordinate parts of this circulation, and inde- 

 pendent of any local causes. Professor Davis seems to have fallen 

 in at first sight, as it were, with this new hypothesis, and says. 

 "Having frequently advocated the sufficiency of the convectional 

 theory of cyclones, I now make haste to place Dr. Hann's obser- 

 vations before the readers of Science, that they may see how 

 clearly a revision of opinion is called for." 



The great facility with which Professor Davis, apparently, can 

 at once change his views on an intricate and perplexing scientific 

 subject, which has puzzled profound thinkers and his haste to 

 forsake his former teaching and to rush into print to acknowledge 

 his past errors, and, with the usual zeal of a new convert, to pro- 

 claim the newly adopted faith, seems very remarkable; so much 

 so, that a suspicion arises that a little supposed high authority in 

 high position has had more to do in the matter than a profound 

 study of physical and mechanical principles. Dr. Hann's memoir 

 was read before the Vienna Academy on April 17, was printed 

 and received in this country by Professor Davis, and his notice of 

 it appeared in Science of May 30 : so it is seen how sudden the 

 transition must have been. 



So far, we merely have Dr. Hann's annunciation of the new 

 hypothesis, without any attempt to form a theory from it by 

 showing in what way the forces which give rise to the general 

 circulation, and which act in two directions only, — toward the 

 poles above, and the revers'e in the lower part of the atmosphere, 

 — can be brought to bear so as to give rise to the subordinate cy- 

 clonic disturbances, and to follow them up in their progressive 

 motions so as to keep up the gyrations. A mere hypothesis, if it 

 is a reasonable one, may be very useful as a basis of work or re- 

 search of any kind, but in itself it proves nothing. If a plausible 

 theory can be built up from the new hypothesis: if it can be 

 shown how, from the general motions of the atmosphere, or the 

 forces upon which they depend, a couple of forces can arise, ne- 

 cessary to give rise to a cyclonic motion, and then how this couple 

 of forces follows after the cyclone to keep it in motion, as 

 the boy with his trundling stick follows after his hoop, and gives 

 it well-directed blows, always in the proper direction; why this 

 couple of forces always tends to give a gyration in one way only 

 in the northern hemisphere, and the contrary in the southern 

 hemisphere ; why the air in the cyclone always ascends and never 

 descends; and so on,— very well. We will wait for this to be 

 done. All this has been done in the condensation theory of cy- 

 clones, with results so satisfactory as to scarcely leave a doubt as 

 to the truth of the whole theory ; and we have a right to claim 

 that as much should be done upon the new hypothesis, before it 

 can be accepted. 



Espy had a cast of mind which was not satisfied with vague 

 general assertions, such as that cyclones are caused by the meet- 

 ing of counter-currents of the air, and that they are continued, 

 and all the powerful mechanical effects are produced, without the 

 expenditure of any energy. And so, in seeking a source of en- 

 ergy, the happy thought occurred to him that this is to be found 

 in the latent heat given out in the condensation of aqueous vapor 

 in the interior of a cyclone, where there is always more or less 

 rainfall. As the air, charged with vapor, is drawn in from all 

 sides below and ascends, the vapor is condensed, and the latent 

 heat given out keeps the ascending air warmer and lighter than 

 the surrounding air. This accounts very satisfactorily for the 

 ascent of air in the interior; and the energy by which the ascent 

 of air and the whole vertical circulation is maintained is laid up 

 in store in the lower part of the atmosphere, through which the 

 cyclone passes, and is not found in the upper poleward-moving 

 currents, as Dr. Hann says, from which there is no imaginable 



way in which it can be brought down and applied to the cyclone 

 in the lower part of the atmosphere. 



The air, being drawn in from all sides below, is pressed toward 

 the right in the northern hemisphere, as is well known, by the 

 deflecting force of the earth's rotation; and so there is a couple of 

 forces, all around, acting in one direction on the one side, and the 

 contrary on the other, which originates and maintains the gyra- 

 tory movement; but the energy spent is all in the latent heat set 

 free, and the deflecting force simply modifies the directions of 

 motion. This accounts also, not only for the gyratory motion, but 

 also for its being always in the same way in the same hemisphere. 

 This very beautiful and satisfactory theory, so briefly sketched 

 here. Professor Davis would have us give up for the merely vague 

 general hypothesis that all depends upon the general circulation, 

 without its having been shown how any of the motions of the cy- 

 clone, as accounted for above, can be accounted for upon this new 

 hypothesis. 



But of this theory Professor Davis says, it "is merely a local 

 application of a theory that is universally accepted to account for 

 the general circulation of the atmosphere between equator and 

 poles ; but the tests now furnished by high-level observations «eem 

 to show that the local application of the theory is incorrect." In 

 the general circulation of the atmosphere the air rises in the 

 equatorial regions where it is warmer and lighter, and sinks 

 down in the polar regions where it is colder and heavier. But, 

 according to Davis, the local application of this principle in the 

 case of cyclones is not correct, but must be reversed ; and the air 

 in a cyclone rises in the interior where it is colder and heavier, 

 and sinks down where it is warmer and lighter, and this is shown 

 by the tests of high-level observations referred to. It is proposed, 

 now, to examine some of these tests. 



One of these high-level tests is found in the temperature obser- 

 vations made at numerous low and high stations among the Alps 

 on Oct. 1. 1889. These gave a temperature of about 4" C, on the 

 average, lower than that of a three-years' normal, for all the sta- 

 tions ranging in altitude from a few hundred metres up to 3,100 

 metres. The argument in this case seems to be this : these tem- 

 peratures are 4° below the three-years' normal; therefore the 

 condensation theory is not a correct theory. In drawing this 

 conclusion, no consideration whatever seems to have been given 

 to the question of what the real requirements of the condensation 

 theory are, and at least two false assumptions have to be made. 

 In the first place, it is assumed that surface temperatures in a 

 cyclone must be above those of the three-years' normal, whereas the 

 conditions of a cyclone have nothing to do with a three-years' normal 

 or any other normal. It is simply required that the temperature 

 of the air in general, over an area of several hundred miles in 

 diameter in the interior of a cyclone, shall be higher than that of 

 the air generally around and outside of this area at the time of 

 the occurrence of a cyclone, so that the heavier air around the 

 cyclone shall force the interior air up, and cause an ascending 

 current. And it is not necessary that even this condition be ful- 

 filled at the earth's surface and at all altitudes, but simply through 

 a certain range of altitude; and this may be, and generally is, up 

 in the cloud regions far above the earth's surface. Say the air 

 below, for a mile or two up, had a lower temperature than the 

 surroundings, but that the necessary cyclone conditions existed 

 above this: thus a vertical circulation and a whirl in the air 

 would take place above, which would diminish the pressure below 

 in the interior, and increase it around about, so that in this way 

 the air below, although having a lower temperature than the sur- 

 roundings, would be brought into the vertical circulation and cy- 

 clonic gyration both by the change in the pressure conditions and 

 by the action of the air above upon it through friction. Of course, 

 in such a case the motions of this lower air would be at the ex- 

 pense of the energy above, and so the whole cyclonic system of 

 motion would not be of a violent chai-acter. This seems to have 

 been the character of the cyclone under consideration. There was 

 only a very moderate barometric depression over a large area, 

 the minimum pressure being 753 millimetres, the winds were gen- 

 tle, and there was some rain and snow. If, therefore, the ob- 

 served temperatures had even been 4° lower than the surrounding 

 temperatures at the same level at a distance of several hundred 



