260 REPORT—1874. 
which the pipe passes shall be always gorged with water when the 
apparatus is in motion. Thus the leakage, if any, which the suction 
in the rear of the pipe creates is satisfied by water instead of air. 
mmm. The brackets, transverse bars, and vertical slides, forming an adjust- 
able framework. 
The details of these arrangements will be readily understood by inspecting 
the drawing, including figs. 2 & 3 (Plate XIII.). 
In the tabulated statement of experimental results (p. 261), the diameter 
of the tube used, the diameter of the pressure-hole, its level above the end 
of the tube, and the immersion of the end of the tube below the surface of 
the water are fully stated. 
It is obvious that, under the arrangement described, the changes of pres- 
sure indicated by the rise and fall of the water in the glass tube include not 
only that due to the difference in the height of the column, but also that due 
to the small variation in the tension of the air within what has been called the 
“*vacuum-chamber.” This circumstance has to be taken account of in the 
interpretation of the observed results, and involves a calculation, which, 
however, is readily made, in terms of the ratio of the diameter of the glass 
tube to the capacity of the vacuum-chamber. Taking account of the 
dimensions of the parts, the correction is made by adding 15 per cent. to the 
observed change of column. This correction has been made throughout in 
framing the table, and the figures there given may be accepted as expres- 
sing the true pressures in terms of head of water at about the temperature 
of 60° Fahr. 
The adaptation of what has been called the water-deck was found to be 
absolutely necessary after a few preliminary trials had been made without it. 
Indeed, as the depth to which the pressure-pipe could be immersed was of 
course limited, it had from the first been. a question how far the pressures 
on the apertures would be affected by the proximity of the free surface of 
the water—since the natural stream-line forces, which would have existed 
in their completeness had the immersion been of unlimited depth, would 
inevitably tend to resolve themselves, to some extent, into some kind of wave- 
motion or surface-disturbance ; and the first few preliminary trials led to the 
suspicion that this cause was producing effects of tangible magnitude, and to 
the belief that they might become very great at high speeds: a trial was 
therefore made at a speed of 900 feet per minute. 
The effect of this speed was so remarkable as to deserve notice, if only as 
affording a striking exhibition of some of the forces inherent in stream-line 
action. 
The end of the pipe was immersed 21 inches, the pipe being 14 inch in 
diameter. 
Immediately in front of the pipe, and embracing its anterior surface, the 
water rose in a thin sheet, which was shattered on the underside of the 
divided disk. In the immediate rear of the pipe the exact state of the water- 
surface could not be very clearly discerned, because the conoidal sheet of water 
which shot upwards from the sides of the pipe, and was broken up by the 
framing of the truck, fell in such a “heavy rain” as to obscure the view; 
probably, however, the water-surface was opened in a deep “ gash” nearly 
to the full depth of the tube’s immersion. . 
The most striking phenomenon was that which appeared at a small distance 
sternward ‘in the wake.” 
At about 3 feet astern of the tube the “gash” had become closed by 
