120 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



companying wind, and the very curious case of waves at a 

 great depth without any visible disturbance at the surface. 

 One of the many points to which he directs attention is the 

 decided development of heat accompanying the undulatory 

 motion of the sea; and, again, he gives the result of his own 

 experiments on the color of the sea, showing that the want 

 of transparency of the water on the Agulhas Bank is due to 

 the disturbance at a depth of two hundred feet caused by the 

 waves of the surface. The fourth section of Cialdi's work 

 deals with the defective methods used in determinins: the ex- 

 istence of ocean currents, especially the use of the log line 

 and float, and gives with great detail the discussion of the 

 phenomena attending Avaves of transportation. He con- 

 cludes that a proper correction for the transportation of the 

 log by the surface action of the waves would cause to disaj)- 

 pear from our maps a vast number of the secondary ocean 

 currents that to-day ornament the nautical charts. 



Finally, Cialdi examines numerous questions in i^ractical 

 hydraulics ; and in the marine constructions that are now 

 going on under his direction, the experienced French engi- 

 neers who are working with him express themselves as con- 

 vinced that the solutions he proposes seem to be the only 

 practicable ones tliat i3romise to secure a permanent triumph 

 over the obstacles that have hitherto j^resented themselves 

 in works of this class. 



The principle of Leonardo da Vinci, which Cialdi says he 

 has made his motto, is so often neglected by deductive phi- 

 losophers, while it is so important to the advance of science, 

 that it is worthy of being repeated here. " We ought," he 

 says, " to begin with experience, and by its means discover 

 the truth. This is the motto to be followed in investiGratins: 

 the phenomena of nature." Hevue Maritime et Coloniale, 

 1873, 96. 



A NEW DETERMINATION OF THE EAETh's DENSITY. 



The determination of the mean density of the earth has 

 been the subject of several long series of observations since 

 the time of Sir Isaac Newton, amonsr which we mav mention 

 Maskelyne's determination by measuring the attraction ex- 

 erted on a pendulum by the neighboring mountain Schehal- 

 lien. Maskelyne's computations gave the earth a density 



