ATKINS ON FISHWAYS. 615 



passes, 12 incbes square. The spiral arrangement occupies an area on 

 the ground of only 13 feet square. This, it should be repeated, is not 

 fitted for use in alewife rivers. The third style of spiral referred to is an 

 adaptation of the common rectangular,* devised by myself lor several 

 dams on the Presnmpscot and Machias Rivers. It may be described as 

 a series of reversed fish-ways, piled one on another; the two arms of a 

 reversed fish-way constituting a circuit. By running forward 15 feeti 

 reversing, and running back the same distance, we accomplish a descent 

 of 3 feet, which enables us to repeat the circuit immediately under the 

 first floor. Thus, whatever the height of the dam, the area occnpied 

 by the fish-way is no larger than for one only 3 feet high. 



A similar statement might be made in reference to other styles of 

 spirals. The saving in ground-area over the extended and reversed 

 arrangements is, therefore, for a dam G feet high, one-half; for a dam 

 9 feet high, two thirds; for a dam 12 feet high, three-fourths, and so 

 on. This saving frequently enables us to occupy a very eligible site, 

 which would be too small for any form but the spiral. The economy of 

 material is not so remarkable, but still it is a very important item. Mr. 

 Pike estimates the saving in his form to be 35 per cent. 



The advantage of the spiral with reference to the location of the out- 

 let (or lower end) has already been alluded to. There is little danger 

 of getting it too far from the dam, on whatever side the water be dis- 

 charged. Still there w ill in most cases be a choice in favor of this or 

 that side, according to the peculiar features of the site. 



E— SUBSIDIAPvY CONSIDERATIONS. 



1. — PROTECTION AGAINST FLOODS. 



The most of our rivers are subject to destructive floods, which, besides 

 the force of the water, assail all exposed structures with floating ice and 

 logs. It is, of course, essential that a fish-way be shielded from dangers 

 of this sort. The first aim should be to select a site where protection is 

 already secured. Frequently, however, it is necessary to place the fish- 

 way in the open river. It may then be protected by a pier, or, if that be 

 impracticable, a uarrow fish-way can be covered with stout plank so 

 that floods may sweep over it without injury. As the latter plan in- 

 volves the entire abandonment of all attempts to regulate the supply of 

 water in the fish-way while the flood continues, it should not be resorted 

 to when it is practicable to secure a position that admits of constant 

 access and regulation. 



2. — 3IATEEIAL AND COST. 



Little need be said in respect to material. By the laws of most of 

 the States where fish-ways are needed, it devolves upon the owner o 

 the dam to build and maintain a fish-way. The question of material and 



* See illustratiou. 



