## Facts

- Eagle Manufacturing Co., as assignee of Edgar A. Wright, owned two patents for improvements in wheeled (straddled-row) cultivators: U.S. Patent No. 222,767 (Dec. 16, 1879) and U.S. Patent No. 242,497 (June 7, 1881).
- The patents concerned a spring mechanism and attachments intended to assist in raising and depressing cultivator plow beams.
- Miller and others sold cultivators manufactured by P. P. Mast & Co.; they did not manufacture the accused machines.
- Eagle sued in equity for infringement in the U.S. Circuit Court for the Southern District of Iowa.
- Defendants asserted (1) invalidity based on prior art and lack of original inventorship, and (2) invalidity of the 1881 patent because it effectively claimed the same invention as the 1879 patent.
- The circuit court upheld validity and found infringement as to certain claims; defendants appealed.

## Issues

1. Whether the 1881 patent was invalid because it claimed an invention already covered by Wright’s earlier 1879 patent (and/or anticipated by prior art).
2. Whether, given the state of the art and proper construction of any valid claims of the 1879 patent, the Mast & Co. cultivators sold by defendants infringed.

## Decision

- The Supreme Court reversed the circuit court’s decree and remanded.
- The Court held the 1881 patent invalid because a patentee cannot obtain a second patent for an invention actually covered by an earlier patent to the same patentee, even if the later claims use different language or broader generality.
- The Court further held that, in light of prior art, the 1879 patent (if valid at all) was limited to the specific spring structure described; as so limited, defendants’ sales of Mast & Co. cultivators did not infringe.

## Legal Principles

- A second patent is void if it claims an invention actually covered by a prior patent, particularly when issued to the same patentee, even if the later claims differ in wording or are broader in form.
- A second patent may be sustained only where it claims subject matter described earlier that is distinct, separable, and independent from what the first patent claimed, including a true improvement that is not the same invention.
- In a crowded field, prior art constrains claim construction; any surviving validity may require restricting claims to the specific structure disclosed.
- The range of equivalents is narrow for incremental (non-primary) inventions; broader equivalency is reserved for inventions that are broad and primary in character with substantially new overall functions.

## Conclusion

The Court invalidated Wright’s later cultivator patent as impermissible same-invention double patenting and, construing the earlier patent narrowly in view of prior art, found no infringement by the differently constructed spring mechanism used in the accused cultivators, requiring reversal of the infringement decree.


Miller v. Eagle Mfg. Co., 151 U.S. 186 (1894)