566 



SCIENCE. 



real. Of the 6000 nebulae hitherto discovered we know 

 nothing of the spectrum of more than 300 or 400, while 

 the observation of all the others with a large horizontal 

 telescope would not be a very formidable undertaking. 

 It would also be interesting to observe the spectra of all 

 the clusters. It is possible that some may consist of stars 

 having singular spectra, or even of disconnected nebulous 

 masses, in fact forming clusters of planetary nebula 3 . 

 The interesting discovery by Dr. Copeland that Burn- 

 ham's double nebula in Cygnus is gaseous, shows the same 

 tendency to aggregation in these bodies as in stars. Ob- 

 servations of the spectra of all the red stars and variables 

 would also probably lead to interesting results. 



4. Photometry. Should the instrument be devoted to 

 photometry numerous problems suggest themselves. 

 Variable stars could be observed near their minimum 

 when too faint to be identified with an equatorial without 

 great loss of time. Faint stars in zones or faint com- 

 panions to bright stars could be measured very rapidly. 

 The relative light of all the asteroids would be an interest- 

 ing problem. Many coarse clusters appear to consist of 

 stars of nearly equal brightness. Their light compared 

 with their distances apart might aid our study of their 

 formation. Another useful investigation would be to 

 measure the brightness of all the nebulas. 



In the application of physics to astronomy doubtless 

 many other problems will suggest themselves. Thus no 

 satisfactory results have been obtained in the attempt to 

 measure the heat of the stars with the tasimeter. The 

 use of this instrument would be vastly simplified if it was 

 placed on a solid pier near the ground, was not moved 

 during the observation, and could be perfectly protected 

 from other changes of temperature than those which it 

 was intended to measure. 



As either of the problems proposed above would occupy 

 the time of a telescope for at least one year, it is obvious 

 that there could be no difficulty in keeping such an instru- 

 ment occupied indefinitely. 



The horizontal mounting is especially adapted to an 

 elevated position, and would permit the use of a telescope 

 where an equatorial mounting would be quite impractica- 

 ble. On the other hand, to an amateur, or for purposes 

 of instruction, an instrument which could be set quickly 

 from one object to another, and where the observers need 

 not be exposed to the cold, would offer many advantages. 

 The impossibility of observing far from the meridian 

 would be less important with a large instrument, where 

 the number of objects to select from is very great. 



There are certain purposes to which this mounting 

 could not be advantageously applied. The study of close 

 double stars and other objects requiring long examination 

 and very perfect definition could be better left to other 

 instruments. The sun, moon, and planets can also 

 generally be better observed off the meridian. If, how- 

 ever, the entire time of an instrument can be employed to 

 advantage, and it can collect several times as much 

 material as an instrument of the usual form, it is no 

 evidence against its trial that there are certain problems 

 to which it cannot be advantageously applied. 



The working force required for such an instrument should 

 consist of at least one observer, an assistant to record, and 

 a number of copyists and computers to prepare the work- 

 ing lists, reduce the observations, prepare them for the 

 press, and read and check the proof-sheets. A large 

 volume of valuable observations could thus be produced 

 every year, which would require at least double the time 

 and money to produce by the same telescope mounted 

 equatorially. The difference in the amount of work will 

 be evident when we compare the number of objects ob- 

 served with a transit instrument per night, with those 

 observed with an equatorial. A hundred objects in vari- 

 ous declinations might be examined in a single evening, 

 while it is seldom that the same number could be identified 

 and measured by an equatorial in a week. 



ON MAXIMUM SYNCHRONOUS GLACIATION * 



By W. J. McGee. 



In the development of knowledge of the cosmos, the 

 tendency has ever been to look at first upon all phe- 

 nomena as mystical and incomprehensible ; and only 

 after repeated observation and much study has it been 

 decided that any class of phenomena may be the result 

 of the operation of the identical laws whose existence is 

 established by every-day observation. Thus, in geology, 

 catastrophism prevailed long, but finally yielded to a 

 rational uniformitarianism ; in general biology the idea 

 of special creation has only given way to that of deriva- 

 tion within the memory of a child ; and in anthropology 

 the mystical view yet generally prevails. The narrow 

 domain of glacial physics, as embodied in the glacial 

 theory, is still in the transitional stage. When that 

 theory was first acceptably propounded-by Agassiz, the 

 details were so vaiied, the recognized relations so unique, 

 and the whole conception so grand and startling, that 

 even the more conservative ot those who early became 

 its advocates, forgot for the time the necessity for keep- 

 ing all assumed data within the bounds of actual obser- 

 vation or legitimate induction ; and hence not the least 

 valuable of the later contributions to the theory are those 

 which bring out its relation to established laws. Such is 

 the aim of the memoirs bearing the above title ; the par- 

 ticular phase of the subject discussed being that known 

 as the "ice-cap theory." 



The conclusion of Tyndall that such a supply of heat 

 as may be necessary to produce large quantities of 

 aqueous vapor, and an area of sufficiently low tempera- 

 ture to not only condense but congeal the vapor brought 

 to it, are the first requisites for glaciation, is adopted at 

 the outset; but it is shown that while the regions which 

 furnish and those which congeal the vapor may be con- 

 tiguous, they must be quite distinct. There is no other 

 substance than water in the solid state which will 

 abstract heat from the superfluitant vapor with such 

 facility as to form, when spread over the surface, a con- 

 denser of sufficient power to meet the requirement of 

 glaciation; and such a condenser must so far exceed in 

 capacity any tax that may ever be placed upon it, that it 

 will immediately condense and congeal all moisture that 

 may be brought to it by aerial currents ; for if the vapor 

 is not immediately condensed it will cut off radiation 

 from the ice below, and thus accelerate melting ; and if 

 the vapor is only condensed but not congealed it will fall 

 as rain, and every pound of it will melt 143 pounds of ice 

 before it is itself frozen. The integrity of the condenser 

 hence depends on its capacity being far in excess of the 

 work it may be called upon to perforin. Now if a con- 

 denser formed of an ice-sheet 1.200 or 1,400 miles in 

 diameter on any part of the globe be assumed, it is mani- 

 fest that the tendency of the accumulating ice will be to 

 form an annular belt of maximum thickness, gradually 

 attenuating toward the center of the area ; for if the 

 vapor-laden air were not immediately robbed of its mois- 

 ture in sweeping over the condenser, the marginal por- 

 tions of the ice would soon be destroyed. But no matter 

 how perfect the condenser, glaciation can never occur 

 unless there are ample quantities of vapor supplied to it ; 

 and the greatest possible accumulation of ice at any lati- 

 tude may accordingly be regarded as proportional to the 

 moisture conveyed thither. It follows that the greatest 

 possible accumulation of ice in polar regions can never 

 have been nearly as vast as that at lower latitudes 

 during the quaternary ; and indeed it was probably never 

 much greater than at present. Geological evidence, so 

 far as accessible, corroborates this view. 



Similar conclusions are reached by an independent line 

 of investigation. Within an extensive area covered by 

 ice or snow, both aerial and aqueous currents would be 

 either stopped or so modified as to be practically inopera- 



* Reprint from Proceedings of the A. A. A. S., Vol. XXIX. 



