This guy, and his decapitated twin across the street, are subway vents. In fact, it turns out that a couple of the mystery towers I've been photographing (specifically, this one and this one) are also subway vents, rather than natural gas line vents (as I originally thought). The same type of vent can be used in different situations, so it's hard to determine just what is being vented unless you happen to know what is underground at a particular location. For more information, and some hilarious misinformation, let's turn things over to the esteemed Kevin Walsh of Forgotten New York.
I love Forgotten New York, it’s so full of details and pictures you can’t find anywhere. I check on it from time to time but I know it will take me many more years to fully digest all the data there.