300 CALCAREOUS MANURES— APPENDIX. 



pond, still it is not done, and the nuisance continues long after it is well 

 known to be such, because there is a contest between t!ie several owners 

 of the pond and of the land covered by it, in regard to their respective 

 shares of profit to be gained by emptying the pond. Many such cases still 

 exist in Virginia; although many of the most unprofitable ponds, from pro- 

 per views of economy, have been drained, and either substituted by cheaper 

 and more efficient canals, or the mills put down entirely. An old inill-pond 

 in Dinwiddle county, which covered 1-JOO acres of land, has been drawn 

 olT, and thereby an indifferent mill exchanged for a large fertile farm. This 

 would not have been done, even if the mill was worthless, but for the 

 ownership of the mill and the land covered by the pond falling into the 

 same hands. There is a mill-pond now kept up in Prince George county, 

 which is supposed to cover nearly 400 acres of land ; and there are many 

 others not much smaller, on different branches of swamps in lower Virginia. 

 The larger the pond, in general, the greater proportion of bottom is left dry 

 in autumn, and the more disease is therefore produced ; and though the 

 draining of such large ponds would be so mucli the more an object of gain, 

 there is the less chance for its being done, because of the many separate 

 ownerships and interests. 



Almost all the mills throughout the lower part of Virginia, and also a 

 large proportion of those in the more hilly middle county, are worked by 

 streams which are inadequate to the daily supply of the mill and evaporation 

 from the pond, even if the grinding is not necessarily suspended or di- 

 minished at any time. To guard against the temporary failure in dry wea- 

 ther, the full " head" of the pond, (or the level of water for which damages 

 were assessed, and to which the water may lawfully be raised,) is much 

 higher than the lowest level that will work the mill. The land covered 

 is also usually very nearly level, so that to raise the water 10 or 15 feet at 

 the dam, will often back the water from one to two miles up the low- 

 grounds. If the variation between a full head of water, and the lowest 

 level, be 5 feet perpendicular, it will often cause the uncovering of many 

 acres of the bottom of the pond to the hot sun, and thereby furnish a most 

 fruitful source of malaria in every such case. Rich alluvial mud, as this al- 

 ways is, thus exposed in hot weather, cannot be otherwise than very inju- 

 rious to health ; and there is not a pond-mill in Virginia, with a variable 

 head, which has not more or less of the pond every summer thus converted 

 to a fruitful seed-bed and nursery of disease. 



Besides this, there is the not rare occurrence of the pond being entirely 

 drawn off in summer, by the breaking of the dam, and its being suffered 

 so to remain for weeks or months, before being again repaired and the 

 pond filled. In this case, a double quantity of bottom is exposed to putre- 

 faction, and fitted for the discharge of unhealthy miasma. 



At all times, in ponds supplied by streams as feeble as most of those 

 used for mills in Virginia, the water approaches to a stagnant state; and 

 therefore of itself is a producer of malaria. In dry seasons, when unus- 

 ually low, the putridity of the water of such ponds fs perceptible to the 

 sense of smell ; and it must be then far from harmless. 



Another, and in certain situations, the greatest evil of mill-ponds, remains 

 to be stated. The others above-mentioned are the effects of the scarcity 

 of the supply of water ; this is from the excess, which is found in all 

 streams, at some times, even though the most deficient at others. 



To guard as much as possible against the expected scarcity of water, the 

 mill-owner aims to hold, when rains increase the usual supply, as "full a 

 head" as he has a right to maintain. When this supply is exceeded, as it 

 frequently is, and greatly, if the dam be not actually broken, and the whole 



