278 CoL Bemifoy on the Resistance of Water ^ [Arnit, 



In the Annals for October, 1815, Prof. Thomson did me the 

 favour to insert a table of experiments on the direct resistance of 

 water to a plane one foot square, and immersed to the mean 

 depth of six feet, but owing to an inadvertency in copying the 

 eighth column, entitled " Exponents of the Minus Pressure," 

 was erroneous, and, therefore, expunged. I have now the 

 pleasure of sending another. Table II, which does not materially 

 differ from the fomier, excepting in the last column. The first 

 contained the resistance of a square plane; the present, the mean 

 resistance of a square and round plane, each containing 144 

 square inches, or one superficial foot. 



Column 1 contains the velocity of the planes through the 

 water in feet per second. 



Column 2, columns of water, the base of each being one foot 

 square, and the respective altitudes equal to the ypace through 

 which a heavy body must fall to acquire the velocity of one, 

 two, or three feet, &c. in a second. 



Column 3, the weights of the different columns of water in 

 lbs. avoirdupoise. 



Column 4, the mean resistance of the two planes in lbs. avoir- 

 dupoise. 



Column 5, the difference between columns 3 and 4. 



Column 6, the minus pressure found by experiment. 



In Column 7 is set down the exponents of the minus pressure. 

 These exponents are found by calculating the various values of 

 m answering to one and two feet, and the corresponding weights 

 •1616 and -6075; then two and three feet, and the correspond- 

 ing weights -6075 and 1*2973, and so on as far as 12 feet. The 

 mean value of the eleven exponents, 1,7646, is then used, and 

 the table extended to 20 feet ; but the same reliance is not to 

 be placed upon the resistance of velocities exceeding 12 feet per 

 second. 



The minus pressure is thus determined : Figures 1 and 2 have 

 the same fore and middle parts ; the bows or foremost extremi- 

 ties wedges each oblique side, measuring three feet, and the base 

 one foot ; the middle part a cube one foot square ; the stern or 

 hinder part of fig. 1 is hkewise a wedge whose obhque sides 

 exceed the foremost by one foot and six inches, these sides 

 being four feet and six inches long. It is evident by inspecting 

 the two figures, that as their fore and middle parts are similar, 

 the difference of resistance, after deducting the friction of the 

 water (which in all cases, the planes excepted, has been done), 

 must proceed from the form of the sterns. The experiments 

 made with these bodies, 1 and 2, are contained in Table 3. To 

 corroborate the above experiments, sohds 3 and 4 were em- 

 ployed ; these, like the former, had the same fore and middle 

 parts, the bows circular, and the centres cubical, but in fig. 4, 

 the wedge stem end was taken away ; therefore the variation in 

 the resistance in this case, as in the former, proceeds from alter- 



