So there I was on the plane, banging away on the laptop Macintosh, when --

What's that? You haven't heard about the new laptop version of the Macintosh, a petite nine-pound package that fits on a tray table in coach but still runs all Mac software, including Finder and Hypercard? Well, there is such a machine. But you won't see it in Apple Computer's advertisements for the Macintosh.

The reason Apple doesn't advertise the new Mac-compatible laptop is that this impressive new PC isn't made by Apple. The laptop Macintosh clone is the first product from a Boulder, Colo., company called Outbound Systems (800-444-4607).

The engineers at Outbound came up with the idea of correcting the several mistakes Apple made when it came out with the first portable Macintosh last fall. Judging from a couple weeks' test drive, Outbound has done a fine job.

Apple's portable Macintosh has been a major market disappointment. It's a handsome machine with some ingenious features. But it's also bigger, heavier, and more expensive (about $6,000 a pop, although retail prices are falling) than almost any other portable.

The Outbound is about 40 percent smaller and lighter than the Mac portable, runs for a couple of hours on battery power with an easy-to-read backlit screen and yet costs barely more than half the price Apple asks for its portable.

Apple has been zealous about bringing legal action against anybody who tries to sell a Macintosh clone. So how did Outbound get away with it?

Outbound has not tried to clone the system software in Apple's copyrighted Read-Only-Memory chip -- the part of the Macintosh that has legal protection. Instead, Outbound uses Apple's own ROMs.

When you buy an Outbound, you also have to have a Macintosh (Plus or SE) of your own. The dealer takes the ROM chips from your Mac and installs them in the Outbound. You can still use your old Macintosh by running a cable from the laptop machine.

If you own a Mac Plus or SE now, you already have the necessary ROMs. Even if you don't, the total price for an Outbound system is lower than a Mac portable. A single-floppy-drive Outbound costs $3,000; the upper-bracket Outbound, with a 40-meg hard disk (but no floppy) costs $4,000. A new Macintosh Plus, which you cannibalize for the ROMS, can be had for $900, and used ones are going for $500. Thus you could buy a floppy-drive Outbound for a total outlay of $3,500.

The Outbound has some good design features. For traveling, it folds up into a single unit smaller than a big-city phone book. To use it, you flip a couple of latches and watch the "Welcome to Macintosh" screen come up almost instantly because the boot-up software is in a fast RAM disk.

The little keyboard communicates with the screen unit through a wireless infrared connection that has worked flawlessly for me. The display is a sharp black-on-white that looks like the screen of an SE. As with many liquid-crystal screens, though, you tend to lose the cursor arrow a lot.

Instead of the trackball, the excellent mouse alternative that Apple put in its portable, Outbound chose to replicate mouse control through something it calls the "Isopoint." This looks like a second space bar below the keyboard. It has a thin roller, which you move up and down, back and forth to steer the cursor. It works, but not as well as Apple's trackball or the Macintosh mouse.

Overall, Outbound's Macintosh-compatible laptop is a fine computer and excellent competitor to the overweight, overpriced portable Mac. I'm eager to see what good idea Outbound hits on next.