May 1, 1896.] 



SCIENCE. 



655 



from one to the other hy delicate grada- 

 tions, each of which must indicate specific 

 physical properties, shows that this is a 

 very rich field of research, which has been 

 only imperfectly cultivated. Many inter- 

 esting conclusions have been developed by 

 observers of such phenomena in the past 

 fifteen j'ears, but onlj^ during the past five 

 years has the conviction become general 

 that this is one of the most important stud- 

 ies for the practical meteorologist. 



With the view of reducing the details to 

 uniformity of method, and to secure cooper- * 

 iition among the observers in different 

 countries, an organization has been com- 

 pleted which will go into effect this spring. 

 A brief history of the movement is as fol- 

 lows : The measurement of cloud heights 

 is an old problem and many devices have 

 been invented for the solution of the prac- 

 tical difiiculties, of which a full account 

 may be found in the Eeport of the Chief 

 Signal Officer, Part 2, 1887, by Prof. Cleve- 

 land Abbe. More or less sj'stematic obser- 

 vations, extending over considerable per- 

 iods of time, have been made at Berlin, 

 Upsala, Storlien, Kew and Blue Hill, 

 (Mass.), by methods depending upon trian- 

 gulation. Besides the simple trigonometric 

 formulse, another system for computing the 

 shortest distance between the two sight 

 lines, devised by Ekholm and Hagstrom, 

 Upsala, also a process for reducing the 

 points on a photograph plate exposed in a 

 photogrammeter by Akerblom, Upsala, 

 have been successfully used and are re- 

 commended as the best known. 



The following are the leading papers on 

 cloud observations : 



1. Mesures des hauteurs et des mouve- 

 ments des nuages, par N. Ekolm et K. L. 

 Hagstrom, Upsala, 1884. 



2. Des Principales mSthodes employees 

 j)our observer et mesurer les nuages, par 

 Hildebrandssou et Hagstrom, Upsala, 1893. 



3. De I'emploi des photogram metres pour 



mesurer la hauteur des nuages, par Aker- 

 blom, Upsala, 1894. 



4. Observations made at the Blue Hill 

 Meteorological Observatory, Annals Har- 

 vard College, Vol. XXX., Part III., by H. 

 H.iClayton and P. S.* Fergusson, 1892. 



At the International conference, Munich, 

 1891, a committee was appointed to con- 

 sider the question of concerted observa- 

 tions on the direction of motion and the 

 height of clouds. This committee recom- 

 mended that observations on the direction 

 of motion and the height of clouds should 

 be commenced at certain stations distri- 

 buted over the globe, and continued for one 

 year; that short instructions be prepared for 

 these observations; that the scheme of cloud 

 classification put forth by M.M. Hildebrands- 

 sou and Abercromby be adopted, and a cloud 

 atlas illustrative thereof be published. 



As the result of these propositions, the 

 cooperative international cloud observations 

 will begin May 1, 1896, and continue one 

 year. As far as known, the theodolite 

 method will be employed at AVashington, 

 D. C, Blue Hill, Mass., and Christiania ; the 

 photogrammeter method at Upsala, Paris, 

 Potsdam, Petersburg, Nijni Novgorod, 

 Manila, Batavia, Melbourne and probably 

 Kew, Calcutta and Sydney. The difficulty 

 in cloud observations is to have two ob- 

 servers, separated by a base line nearly one 

 mile long, set their sight lines on exactly 

 the same point of a rapidly moving and dis- 

 solving cloud. The advantages of the the- 

 odolites is that the instruments are cheaper, 

 many more "observations can be taken with 

 the same labor and the calculations are the 

 briefest possible by any method. The ob- 

 servations that must be rejected at the out- 

 set can be determined by a small plotting 

 machine, being a model of the real base line 

 and instruments, such as invented by H. H. 

 Clayton, at Blue Hill. Photographs, on the 

 other hand, possess the advantages of giving 

 definitely the point on the cloud, but the 



