190 ANNUAL OF SCIENTIFIC DISCOVERY. 



0.13 inch, and for the latter, 0.05 inch. In order, therefore, to reduce the 

 pumping, so that the probable error of an observation from this cause 

 may not exceed 0.01 inch, the contraction should be to 20 minutes at 

 least. 



4th. It appears to be very desirable that each ship should be furnished 

 with two barometers^-one for calmer weather and the other for rougher 



o 



the former having the tube contracted to 10 or 12 minutes, and the latter 

 to about 25* minutes. This would render good observation obtainable in 

 all states of the weather ; and if occasional comparisons of the two were 

 taken, would, besides obviating to some extent the inconvenience arising 

 from an accident to one, afford the means of checking any changes which 

 might occur in the zero points of either instrument. If, however, t\vo 

 barometers cannot be supplied to each ship, I am disposed at present to 

 think that a contraction to about 15 or 20 minutes would be generally the 



most convenient." 







ON THE CARTESIAN BAROMETER. 



The following remarks on the Cartesian Barometer were recently made 

 before the Royal Institution, England, by Dr. Roxburgh : 



Soon after the discovery of the variations in the height of the barometer, 

 Descartes proposed the following mode of rendering them more conspicu- 

 oxis, almost as much so as they are in one filled with water alone. He 

 suggested that two tubes should be joined to the opposite ends of a short, 

 wide cylinder, so as to form one straight tube, which, "being closed at one 

 end, was to be filled with pure water and mercury in such proportions as 

 to allow of the two fluids at all pressures meeting in the cylinder. In 

 this, the Cartesian barometer, the pressure of the atmosphere is balanced 

 by the water and mercury conjointly : but the variations of pressure are 

 indicated chiefly by movements of the water, as the level of the mercury 

 varies little in consequence of the large area of the cylinder. The move- 

 ments of the water and mercury are to each other inversely as the areas of 

 the tube and cylinder. The scale is that of the common barometer 

 enlarged, as in the wheel barometer ; when, therefore, the movements 

 are said to amount to so many hundredths of an inch, it is to be under- 

 stood as meaning that they are equal in value to that height of mercury. 

 The scale can be enlarged so as to render movements of one four- 

 hundredth of an inch visible to the unassisted eye. The only records of 

 this instrument that I have seen state that the air contained in the water 

 is given off when the pressure is removed, and so renders its indications 

 incorrect ; also that this imperfection is irremediable. This depression, 

 amounting in one year, in my first experiment, to only .02 of an inch, has 

 led me to suppose that the depression which caused the plan to be set 

 aside was owing to the force of vapor, which was not so well under- 

 stood at that time as at present ; and as many variations of pressure are 

 easily seen in this barometer, which would escape notice in the mercurial 



