70 



SCIENCE, 



[Vol. IV., No. 76. 



plateaus (whether high or low) , on the coast, or 

 in different seasons ; and it would seem from 

 these records that the only means of obliterat- 

 ing the effect of these temporary disturbances 

 is in having a large number of observations 

 made, and taking their average. 



This would also seem to throw some doubt 

 upon the value of Williamson's method of 

 obtaining this correction from the curve of the 

 day : for days differ so much that a large 

 arbitrary or constructive element would be in- 

 troduced, thereb}' damaging the results as far 

 as scientific accuracy is concerned ; for we can 

 easily see that the difference between a clear, 

 bright da\% and a da} T in which the sky might 

 be wholly or parti}- covered, would be great, 

 and therefore much would be left to the judg- 

 ment of the observer with regard to applying 

 the whole or onlv a part of any correction 

 which might be obtained for use in such cases : 1 

 for, at best, we only get a sort of skeleton aver- 

 age from the mathematical formulae ; and it is 

 quite certain that the formula of Laplace gives 

 too high results for general use, since it is onlv 

 adapted for the summer months. 



The second objection to this method, there- 

 fore, would seem to be found in the compara- 

 tively arbitrary use of the data obtained. 



If we could only obtain a partial solution 

 of this problem by securing a portion of the 

 proper correction, it would be a great gain. 



The temperature correction would not, per- 

 haps, be so difficult to obtain; but, when we 

 once introduce the element of moisture, we 

 perceive that the uncertainty of controlling the 

 conditions increases, except by having full 

 averages of all the elements involved, as their 

 variability is so great. 



One has only to look into the work of Plan- 

 tamour to become convinced of this ; and the 

 results obtained from these records, as worked 

 out by Dr. Guyot, show it conclusively. 



It has therefore been Dr. Guyot' s principle 

 to simplify the methods, both of observation 

 and of computation, as much as possible, so as 

 to facilitate the making of a large number of 

 observations, and from their averages obtain- 

 ing better results. B}' this means the original 

 records are touched as little as possible ; and 

 the results are sufficiently accurate for the 

 purposes of the geographer. 



The trouble is, that these difficulties arise 

 from the nature of the problem ; and it is very 

 doubtful whether the}' will ever be completely 

 mastered. 



There is another objection to the proposed 

 method, which would seem vital ; and that is, 



1 Bulletin No. 2, E. M. museum of geology and archaeology. 



that it materially increases the cost of such a 

 survey, not only by the emplo} r ment of an 

 additional observer, but also by making the 

 measurement of a number of vertical base- 

 lines b}' level a necessit} 7 in an extended 

 survey ; and any thing which does this, thereby 

 bringing the expenses nearer to the amount 

 necessary for a regular levelling-survey without 

 proportionally increasing the accuracy of the 

 results obtained (for the liabilhVv to errors of 

 observation and computation is greater) , is at 

 least of questionable advantage. 



In his w Non-marine fossil Mollusca,' Dr. C. 

 A. White presents a valuable .review of the 

 North-American brackish, fresh-water, and 

 land Mollusca, beginning with their first repre- 

 sentatives in the Devonian, and tracing the 

 history of many cases of persistence to modern 

 forms. Every paleontologist has to contend 

 with large gaps in material ; and these are more 

 extensive among the inland than among the 

 marine Mollusca. Another difficulty lies in 

 distinguishing between the brackish and fresh 

 and salt water forms. These are often com- 

 mingled, especially in the Laramie rocks, which 

 bear evidences of frequent littoral changes in 

 the estuaries and inland seas. Following a 

 geological introduction, the author presents an 

 elaborate historical catalogue of the genera of 

 the Conchifera and Gasteropoda. There is a 

 happy omission of any attempt at revision of 

 species (an endless task) , and little technical 

 description ; while more attention than usual is 

 given to the interesting question of changes of 

 habitat during successive periods, and relation- 

 ships with other fossil and modern forms. A 

 curious fact, which has so many parallels among 

 the other Mollusca, is the early appearance and 

 development of the pulmonate gasteropods, 

 which are found as low as any of the Conchifera. 

 With the necessary presumption of the deriva- 

 tion of the non-marine from the marine types, 

 the author infers that this has taken place not 

 only in paleozoic, but in more recent geologi- 

 cal times. The marine types, having suffered 

 the fewest changes of environment, have been 

 the most persistent ; then rank the land and 

 brackish-water types ; but, in view of the con- 

 tinual changes in the fresh- water areas, the 

 persistence, even to the present day, of several 

 fresh-water forms, is most remarkable. Ac- 

 cording to the author, the latter forms abounded 

 in the great tertiary lakes : they survived the 

 contraction of the lakes into the great river- 

 systems, and they owe their wide dispersal to 

 the confluence of these river- systems, as in the 

 case of the Ohio and Mississippi, which origi- 

 nally poured into the Gulf. It appears that 



