22 REPORT — 1851. 



have yet been made, the personal equation between diflferent observers, if not entirely 

 insensible, is at least confined to a few hundredths of a second. 



It is through the facilities and means furnished by the Coast Survey Department 

 of the United States, under the superintendence of Dr. A. D. Bache, that individuals 

 there have been enabled to bring to its present stage the application of electro-mag- 

 netism to the purposes of geodesy and of astronomy ; it having been at the expense 

 of that department, and frequently by its officers, that nearly all the experiments 

 have been conducted. 



Daguerreotypes of the moon were shown to the Members of the Section, taken by 

 Messrs. Whipple and Jones of Boston, from the image formed in the focus of the 

 great Equatorial of the Cambridge United States Observatory. 



On a Method of Sounding in Deep Seas. By J. P. Joule, F.R.S. 



The impossibilitj' of sounding the depth of the ocean by the ordinary plumb-line 

 has been remarked for nearly three centuries, and during that time numerous inven- 

 tions have been made in order to measure great depths at sea. The instrument 

 which appeared most feasible consisted of two bodies, which in connexion with one 

 another, sank to the bottom, where, becoming detached, the body of lighter specific 

 gravity rose to the surface. The time occupied indicated the depth of the water, 

 which was otherwise ascertained by vanes propelled by the water during the motion 

 of the sounding apparatus through it. 



In these instruments, the body which was intended to float to the surface was 

 generally of wood. But, beyond a very limited depth, an instrument composed of 

 such a material would fail to give any result ; for Scoresby has shown that when 

 wood is sunk in a deep sea, it becomes so saturated with water as to become of 

 greater specific gravity than that fluid. To overcome this difficulty, the wood has 

 been covered with pitch, &c.; but it may be doubted whether such a coating would 

 prevent the penetration of water under great pressure. I find, moreover, that light 

 wood, cork, &c., when subjected to a pressure of some tons on the square inch, are 

 crushed so as to become specifically heavier than water ; and that they remain so even 

 after the pressure has been removed. A wooden float would therefore be crushed, 

 even if the coating of pitch were sufficient to keep out the water from its pores. 



A method of overcoming this difficulty has been recently devised by M. Faye, in 

 which he substitutes a vessel of sheet-steel filled with oil, or other light inelastic 

 fluid, for the wooden float. M. Faye recommends the use of a cylinder of sheet- 

 steel 1 metre high, and 2 decimetres in diameter, which, filled with potato oil, 

 would have a specific gravity of 0"88 in comparison with sea- water, and a force of 

 ascension equal to 15 kilogrammes. For further details of his apparatus, see the 

 ' Comptes Rendus,' January 20. 



I believe it to be impossible to improve upon the general principle adopted by M. 

 Faye, but it has occurred tome that a great improvement in the detail would be effected 

 by substituting for the metallic vessel in his apparatus, one of gutta percha, which 

 if filled with alcohol or a light oil, need not exceed the gravity of 0'8, that of sea- 

 water being called unity. In this case the ascending force would be nearly double 

 that of M. Faye's instrument. As a further means of increasing the velocity of 

 ascension, the gutta percha vessel should be constructed on Mr. Russell's wave prin- 

 ciple. If the float were 10 feet long, and 1 foot in its greatest diameter, and fur- 

 nished with a proper weight, a depth of 7 miles might be sounded in less than one 

 hour. 



Dr. Faraday submitted, on the part of Dr. Roxburgh, a specimen of dark glass to 

 the examination of the Section under the following circumstances : — 



It had been employed to darken the image of the sun outside the eye-piece of a 

 telescope of 3 inches aperture and 47 inches focal length ; the magnifying power of 

 the eye-piece was 100, and the dark glass was placed at the distance of about one- 

 eighth of an inch from it. No other eye- piece produced the same effect. In about 

 two minutes after the telescope had been directed to the sun, and the focus adjusted, 

 the greater part of the field of view became dim, as if the glass had been breathed 

 on, and the definition became suddenly destroyed. When the glass was examined 



