326 
POPFLAK SCIENCE EEYIEW. 
by tbe Fraunhofer lines, leading, it is hoped, to the discovery of a combina- 
tion of crown and flint glass which shall be free from secondary spectrum or 
absolutely achromatic. The result of this investigation will be the subject 
of a future communication.’’ 
PHYSICS. 
What is a Tachymeter f — This question is answered by a paragraph in a 
recent number of “ Les Mondes,” in which M. Dupont gives an account of 
an instrument which appears to be devised for the purpose of rendering the 
teaching of geometry a rapid and pleasant course at schools. M. Lagout has 
given some lectures to prove this at Clermont-Ferrand, the result being that 
the Maire and the Recteiir have decided that tachymetry shall be introduced 
into the schools under their control. The instrument is made in three 
different sizes, at the price respectively of 10, 21, and 100 francs ; it is for 
sale at 41, Rue J. J. Rousseau, Paris. 
Temperature of Arctic Sea near Spitzbergen. — Captain X C. Wells gives 
the results of his experience on this subject to the “ Proceedings of the 
Royal Society ” (141). He records a very curious result, viz., gradual increase 
of temperature at great depth. On July 12th, when in 80° 17' N., and 
when the vessel was fixed in the ice, the temperature gradually increased to 
64° F. at a depth of 600 fathoms. These facts indicate the southward flow 
of a vast body of warm water. It cannot be said that the heat is derived 
from the Gulf-stream, because nowhere in its course, even in such latitudes 
as 60° or 60°, does it acquire so high a temperature, even at the surface ; 
and it is highly improbable that the general warmth of the ocean along the 
west coasts of North Europe, on the shores of Norway, could possibly be 
supplied by the limited body of warm water which leaves the Gulf of 
Florida. If the whole of the Gulf-stream water were spread over the 
warm- water area in the north, its depth, even allowing the most liberal 
estimate for its volume, would not exceed 10 fathoms; whereas warm 
water of 42° F. occurs to the depth of 400 fathoms in this region, and north 
of Spitzbergen it is found as high as 64° F. at 600 fathoms. If it be said 
that this temperature is due to the northward drifting of the Atlantic 
from warmer localities, we are met by two difficulties, of which one is, that 
the soundings obtained by Carpenter and others gave temperatures much 
below 64°, and the other is, that the waters flow south, not north. Volcanic 
action, or a warm mineral spring rising from the ocean-bottom, may by 
some be imagined to be the cause of the temperature of 64° ; but there is 
no evidence of either of these agencies, and it is quite reasonable to suppose 
any other feasible cause. Passing over the discovery of 64° F. at this 
depth, we still have to account for the water of 42° F. flowing southwards 
as evidenced by the increase of its temperature as we proceeded northwards. 
Wave-lengths of Sound in Gases. — Professor S. M. Mayer says that he has 
hung up in his lecture-room before the students a series of gum-tubes having 
lengths of 1, 1^, 2, 2^, 3, &c. wave-lengths of different notes. The tubes 
forming any one of these series, are used with the organ-pipe and resonator 
corresponding to their note ; and as they are successively adapted to the 
