Italian Irrigation. 
199 
tion, the tromha copcrta, or covered chamber. It has, in the 
established form of the modulo, a fixed length of 10 hraccia, 
equal to very nearly 20 English feet, and a breadth variable 
according to the size of the head-sluice, which it exceeds by the 
fixed quantity of 5 local- inches on each side, or 10 on the entire 
bi'eadth, being nearly 1'64 English feet. The bottom of the 
covered chamber, D H, Fig. 2, is formed with a slope to the 
rear, or as a ramp : the height of this slope, H h, is equal to 
8 local inches, or 15| English inches very nearly ; and its object 
is to diminish the velocity with which the water reaches the 
measuring outlet G H. Farther to assist in effecting this object, 
the perfect mcdulo is provided with a horizontal top of stone 
slabs, or planks, called the cielo morto, the under surface of 
which is at precisely the same height as the water ought to have 
over the outlet G H, so as to secure the fixed discharge ; that is, 
3"93 English inches above the upper edge of G H. The great 
purpose of the apparatus being to secure the discharge taking 
place under simple pressure, the cielo morto, which may bo 
roughly rendered the deadeniiu/ cover, is found to reduce the 
irregular motion of the water in passing from the sluice A B to 
the measuring outlet G H. 
" To admit of ready inspection of the state of the water within 
the covered chamber, the following arrangements are made : — 
The entrance to the chamber is covered by a stone slab of con- 
venient thickness, shown in section at E, Fig. 2, the lower 
surface of Avhich is precisely on the same level as the upper edge 
of the outlet G H. The heighi of the ramp H li being 15"72, 
and that of the outlet G H being 7"86, the surface of the slab at 
E should be just 23*58 English inches above the sill of the head 
C D. An open groove, L D, is made in the masonry, large 
enough to admit a graduated rod or measure ; and when the 
water stands at a height of (23'58 + 3'93) 27'51 inches above the 
sill at D, it is known that the proper head of pressure exists at 
G H. As it is found to be greater or less, the sluice is raised or 
depressed, so as to adjust the pressure to the fixed standard. 
"The slab of stone in which the measuring outlet is cut being 
fixed at G H, Figs. 1 and 2, immediately in rear of it there is 
placed the tromha scoperta, or open chamber. Its breadth at N, 
Fig. 1, is two local inches, or 3'93 English inches greater on 
each side than that of the measuring outlet ; or in all, 7"86 inches. . 
Its total length, N O, is very nearly 17| English feet. Its side- 
walls, which are perpendicular like those of the covered chamber, 
have a splay outwards, so that the breadth at O is 11-79 inches 
greater than at N, or 15-72 inches in excess of that of the regu- 
lating outlet G H, being the same as that of the covered chamber 
throughout. To insure the free run of the water from G H, the 
