Remote PowerShell Defrag

I'm trying to setup remote defrag on some servers. I've checked a bunch of examples on the Google - but I don't quite understand the basic command to call to get defrag to run.

Can someone please tell me what I am missing from this code:

$Servers = "ServerName"

ForEach ($Server in $Servers)
{
    $Drive = Get-WmiObject -Class Win32_Volume -ComputerName $Server -Filter "DriveLetter = 'c:'"
    $Drive.Defrag()   
}

Thanks for any help.